I got the depo-provera shot feb 25th. i'm supposed to go back may13th-27th. i had protected sex today may 18th. is the birth control still in my system?

Fine: You should be fine. I count 12 weeks from February 25th to be this coming Tuesday, May 20th. So, yes, the Depot Provera (medroxyprogesterone) should still be preventing pregnancy. Having said that, no birth control is ever 100%. Depot Provera (medroxyprogesterone), however, when taken regularly, is very good at preventing pregnancy. Make sure to get your next shot right away!