It also sounds like proctosigmoiditis, this would include the rectum and sigmoid, I think this is what the doc meant by rectosigmoid (never heard that wording before. This is also what I have, I usually just say UC, it's easier :)
And as far as it spreading...well, you will learn this is a very individualistic disease, what happens to one won't happen to another. Some people here will tell you their colitis never spread and some will have gone from proctitis to pancolitis in just a few years.
The meds will help! I've not read all your threads but are you on a oral med in addition to the Rowasa? Lots of people prefer an oral/rectal combo to really combat the inflammation to to possible stop spreading.
Ask as many questions as you want, someone will have an answer!!
Amy, 33, dx with idiopathic colitis July 2009, confirmed UC Sept 2009
Currently: In Remission since April 2010
Asacol 4 pills 2 times daily
Rowasa (slow taper, skipping every 3rd night)
Probiotic: Not taking one now, experimenting