So you're 100% sure you're not pregnant, right? Sorry to ask, just wanting to make sure!
If you really don't want to be on the pill, I suggest you try charting your cycle. Then you'll know if you are or aren't ovulating, which will help your doctor determine why you're not having a period. A good book on charting is Taking Charge of Your Fertility. There's also a website called fertilityfriend.com that covers the basics and can get you started with charting.
That said, it can take a few months for a woman's cycle to regulate after coming off the pill, so this all could be because of that. Also, though, why did you quit taking the pill? If it seems to help keep your UC in check, maybe it's a good idea to keep taking it.
Symptoms began in November 2008, ~4 weeks after giving birth to my son.
Eased for ~3 weeks in December, possibly b/c of probiotic use?
Returned in January 2009 (with a vengeance), diagnosed with pancolitis on January 30.
Currently taking Asacol (400mg 4 pills 3x daily), Rowasa every other night, Culturelle probiotic, and Zoloft (25 mg).