I've had mild left sided UC ever since I was diagnosed 10 years ago. When I would flare it was often, but not always, accompanied by D. And my flaring usually followed me not taking my meds religiously (rectal 5asa and oral asacol).
A few months ago, after a round of antibiotics, I started to have symptoms that you describe. I let it go on for a few weeks. After a colonoscopy, the dr told me that my left side had become severely inflamed and my right side was now mildly inflamed. Not what I expected or wanted. I think that my UC was active before the antibiotics, but the antibiotics disturbed whatever fine balance I had going on.
My advice is to call your GI dr. I hope that your symptoms will subside on their own but if it is a flare, you should try to catch it early on. It may also be time to discuss with you dr whether or not your current treatment plan is working for you (something that I did not do enough of).