Good afternoon - I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with dogs and cats. I'm sorry to hear that your pup isn't feeling well - I'm here to help. We can chat back and forth here until I've answered all of your questions and you're satisfied. Sometimes we need to talk a bit to clear up all of your concerns.
Did the vet think that your pet was constipated? The reason I ask is that whipworm classically causes what we call colitis. Colitis is inflammation of the colon and it causes dogs to have the intense urge to squat to poop, but usually since they have diarrhea, only a very small amount of feces actually comes out. This is in stark contrast to true constipation, in which the feces are hard/firm, difficult to pass, and are backing up in the colon because they're not able to push them out.
If you've been seeing small frequent amounts of liquid stool, plus/minus blood or mucus, this is most consistent with colitis. The medications most vets prescribe for colitis would help the inflammation in the colon and help settle the issue, but sometimes it can take a few days. I typically use a medication called metronidazole that is a bit anti-inflammatory to the colon and helps firm up diarrhea, a thorough deworming, and a good quality probiotic like FortiFlora. Probiotics can help diarrhea cases tremendously. As you can imagine, if they have diarrhea or loose stools, that's different than being truly constipated. If a pet is truly constipated, I usually start with a stool softener like lactulose. Obviously we wouldn't want to give a stool softener to a dog with colitis and diarrhea, as that would only make the situation worse :)
Also, as a vet, I always want to know if my patients aren't recovering the way I'd expect, so I would call your vet when they are in the office next to let them know what's happening and get some guidance from them. Hopefully this is helpful info - what other questions can I handle for you?