Can you tell where the fluid is coming from? I am guessing that it is the vulva from the rest of your question, and that would indeed be the most likely source. Can you be more specific about the situation in which you find the fluid? Are you seeing a wet spot when she gets up? If so, look at her backend immediately to try to see if it is from the vulva or the anus. Can you actually see the fluid dripping from her? I have also seen dogs that pass a little bit of fluid from their anal glands and then lick, lick, lick at where the fluid landed and the owner finds the big wet spot from licking. Anal gland fluid is pretty smelly, but the area where they are licking may not retain the smell. Can you tell me what was done for the $150 and what medication she was put on?
I think your first instinct that it was urine may be correct. Urinary incontinence is pretty common in spayed female dogs and if the urine is fairly dilute, it may not have much smell or color to it. Luckily, there is medication for this, although it is a good idea to have a urine sample checked first to make sure there is no evidence of infection.
In case your sister didn't go into any details, a stump pyometra is when the small amount of the uterus that was left after the spay surgery becomes inflammed or infected. I would expect that the discharge would be more like mucus rather than clear watery discharge with this condition. The best way to diagnose this condition is with ultrasound of the abdomen.
Vaginitis could also account for some chronic discharge from the back end. She may be quite inflammed around the vulva in severe cases. Again, more of a mucus discharge would be expected.
Definitely try to pin point where the fluid is coming from if you have not already done so. If you can be certain it is from the vulva, that will help your vet know what type of conditions to check for. Also let them know if you have seen her licking her back end a lot.
It sounds like you are not very happy with your vet at this point and I understand that. Perhaps it is time to try a different clinic. You might try asking any of your friends who have pets which vet they use to find a new place to go. A urine should be checked to help rule out a urinary tract infection and possibly some blood work would be in order, especially if you have noticed any change in her water consumption or if any abnormalities are found on physical exam. Sometimes x-rays may be recommended depending on the results of the above mentioned tests.
If you can be more specific about the situations in which you are finding the fluid, I may be able to help you better.
This very much sounds like urinary incontinence, which is very common in spayed female dogs. I don't have an explanation for why this did not seem obvious to your first vet with that description, so I would agree that it is time to go somewhere else. Be sure to mention that wet spot after getting up as that is the classic symptom for urinary incontinence.
If you are noticing an increase in water consumption, be sure to mention that to your vet as that could suggest something more serious, but this would be much less likely in a 1.5 year old dog. If this is present, your vet will want to do some blood work.
Urinary incontinence occurs when the sphincter muscle that keeps the urinary bladder closed is not as strong as it needs to be so urine leaks out, especially when they are sleeping. This does make them more prone to urinary tract infections so your new vet should check a urine sample - When you make the appointment, ask if they would like you to bring a urine sample with you if possible. If not (some vets like to collect a sample themselves as this allows them to do a culture if needed), be sure to keep your dog inside for several hours prior to the appointment and don't let her urinate on the way into the clinic so they can get the sample they need.
There are two medications that are commonly used to treat urinary incontinence. DES is a estrogen hormone that used to be used all the time. It is inexpensive and convenient as it is usually only used once or twice a week (after an initialy daily starting regimen). Down side is it has some uncommon but serious potential side effects. Because of this, many vets will use phenylpropanolamine as their first choice. This is more expensive and is usually dosed 2 to 3 times a day (may be used for the rest of the dog's life) so it is less convenient, but it is safer. A few dogs will require both meds, and a VERY few dogs do not respond and other approaches have to be considered.
I agree that Angell is an EXCELLENT facility but there are a lot of good vets in other places too. Keep going until you find one that you can work with. As a general rule, larger clinics (multiple vets) are often more progressive clinics, but there are plenty of exceptions to this as it will depend on the individual vets involved.
I hope this gives you the information you need so you can get Maggie started on the appropriate medication as soon as possible. Please let me know if you have additional questions.