Is it all your opinion that diet has nothing to do with this disease? Could three weeks of doxycycline have caused it in my case?
No, most people around will tell you that diet is a factor, and many will tell you that it is a huge factor. I think diet helps some people more than others. It's helped me a lot. There are a lot of diet modifications you could try if you want. The Specific Carbohydrate Diet and the Paleo Diet get some pretty good press. Looks like doxycycline is an antibiotic and antibiotics are indeed a trigger for UC. Plenty of people get UC after or during a course of antibiotics.
Others have answered your original question well. Hydrocortisone suppositories or enemas (these are steroids to get the disease under control fast but cannot be used long-term) and Rowasa enemas (not as powerful as steroids but safe for long-term use) are available as generic so aren't too expensive. There's also sulfasalazine but does that come in suppository-form? I think that's only oral and thus not as useful for proctitis. But it's also pretty inexpensive.