I'm only guessing here (I have no medical knowledge!), but I get the impression from your post that the vet felt that colitis was the most likely cause, but doesn't know for sure that it is colitis. Perhaps the tests to find out if a dog actually has colitis are expensive, invasive, stressful, painful, or something like that. Given the dog's age and other conditions, they may have wanted to avoid anything stressful if possible. So they decided to try treating it as colitis not only because they expected it to help, but if it did help, that would help confirm the diagnosis.
I have no idea how quickly the colitis medicine should make your dog feel better. I would at least call the vet asap to tell them how your dog is doing, and ask what you should do next. Maybe the answer is that the medicine seems to be working, but will take more time, and maybe they can prescribe some pain meds. Maybe the answer is that the medicine should have had more effect by now, so it may be something other than colitis. Maybe now they will want to do some tests, or try a different treatment. Maybe they will want to send you to a specialist.
Your dog is older, and has other health problems as well. I'm sure you're very worried about her. As someone who's been through a similar situation with a beloved dog, I know it can be difficult to take in all the information during a vet visit. We might be calm and collected in a (human) doctors office, asking all the right questions, but fall apart when our pets are involved.
When you go to the vet, it's helpful to have a written list of all the symptoms you're worried about, and the questions you want to ask. When the vet recommends a treatment, you'll probably want to ask:
- How quickly should I expect to see some improvement?
- How much improvement can I expect (will the problem go away, or will it need continual management)?
- Are there additional symptoms I should watch out for?
- Considering my dog's age and other medical conditions, I'm particularly concerned about her quality of life. With that in mind, do you still think this is the best treatment?
I hope your dog will have many more happy years with you, and don't want to inject a note of doom and gloom. It's just that once a dog gets to a certain age and develops a few medical conditions, it's good to start thinking ahead. Think about what activities your dog enjoys, and how to maximise the number of good days, rather than maximising her lifespan.