Ok - that's great!
What you are describing may well be a "vaccine reaction" which is an adverse reaction to vaccination.
Some dogs are a little bit sleepy for a day or so after vaccination, and some will have a severe allergic reaction immediately on injection (like humans who are allergic to nuts - it can be life-threatening).
Your dog seems to have had a reaction that is somewhere in between these two extremes. This is not the mildest form of vaccine reaction that she has had, but not the worst either.
I agree totally with your veterinarian that giving Benadryl (diphenhydramine) should help her. The dose that one generally gives is 1-2mg/lb. It comes as 25 mg tablets. This dose can be repeated every 8 hours for 24-36h.
Here is more about Benadryl:
You should see improvement within 30 minutes. If you do not see improvement, or if you see swelling, diarrhea or vomiting then I strongly recommend that you take your dog immediately to the closest veterinary hospital for treatment. Untreated, it is possible for a dog to have a life-threatening reaction to a vaccine!! They can go into shock.
Although I agree with your vet that Leptospirosis is a risk for a dog that spends a lot of time outdoors, it is also one of the vaccines that is most likely to cause a vaccine reaction.
Something to keep in mind if this is a vaccine reaction is what to do about future vaccines! She may never have a reaction again, or she may have the same or even worse reactions. But there are steps you can take to help prevent problems.
I strongly believe that vaccines are beneficial, and you should continue to vaccinate to protect your baby from serious disease. But at the same time, there are some things that you can do to help prevent this from happening again.
1. Mention it to your vet.
If I were to give another vaccination to a dog that had reacted this way the first time, I would consider "pre-treating" the patient with Benadryl or an anti-inflammatory like Dexamethasone before giving the vaccine. This helps prevent allergic reactions before they start, and usually resolves the problem.
2. Schedule your vaccine appointment with thought.
You are best to schedule your pet's next vaccine appointment early in the day when someone can be home to monitor her for the following 8 hours, or leave her at the clinic after her vaccination to be monitored there. Be careful with Saturday appointments - many clinics close at noon, and if your girl has a severe vaccine reaction you may end up at an emergency clinic!
3. Separate all vaccines.
I would make a point of avoiding giving more than one vaccine at a time in this dog. For example, don't give the Distemper vaccination on the same day as the Rabies vaccination as this may be too much for her immune system, and she will be more likely to have a reaction.
4. Make sure that you always tell the vet every time this dog is due for vaccines that she had a vaccine reaction previously. That way appropriate precautions can be taken to avoid this in future!
Generally, when I see dogs with this problem, I treat with an injection of corticosteroids to stop the allergic reaction. I would also suggest that you keep your girl quiet, and offer her frequent, small amounts of clear liquids. You can offer her a bit of onion -free, chicken or beef broth, or the water from a can of tuna - all of these diluted 50:50 with regular water. You can offer Gatorade or apple juice diluted with water. In a dog this size, offer about 1/2 cup every 30minutes.
Here is more information about vaccines and reactions:
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The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.