Brownish discharge a few times a month=spotting.
You said that these changes sort of coincided w/ when you became a vegetarian. According to what i've read, sometimes dietary changes can cause your period to be altered--even stop entirely.
Another thing to keep in mind is that you're 16 years old, and you're still growing. Many girls your age have highly irregular periods.
I suggest talking w/ your physician about it, he/she will be able to give you the best advice.