I've missed my depo-provera shot in dec. had unprotected sex in january, and still haven't had a period could i be pregnant?

Yes.: Check a urine pregnancy test if your period is more than 1 day late (90% sensitive) or more than 1 week late (>99% sensitive). Take Folic Acid 1-5 mg daily and see your gynecologist for contraception and prenatal care as needed. Best of luck!

Missed Depo: The only way to determine if you are pregnant, even in the best of circumstances (timely depo, iud, tubal ligation, ocps, etc) is to have a pregnancy test asap. This could be urine (cheap and accurate) and blood HCG level (the gold standard) its worth doing a urine test every 48 hours as the HCG levels in the blood double in that time and the numbers rise fast enough that any urine preg test works.