Last night i began to have urinary pain and shortly after started peeing blood. i have had many utis, but blood has never been a symptom for me. i should begin my period tomorrow, but i am sexually active. is it more likely this is a sign of a uti or is i

Probable from UTI: Blood in the urine can occur with UTI and since you are having painful urination then blood probably is from your bladder. You need to see your doctor to get checked and treated.

Probable UTI: Urine is sometimes bloody during a UTI, and rarely due to STD. If your symptoms are otherwise typical for past UTIs, that's probably what you have. But the fact that STDs are on your mind indicate you should be tested for gonorrhea and chlamydia, which should be done at least once a year anyway for sexually active persons, especially if not mongamous. Better safe than sorry!