Can i really be pregnant?

I dont really know what to think here, me and my boyfriend are very care on calculations or the grace days. My time of the month is almost clock work i alway start eigther on the 18-20 always, well we had on the 15th i should not have been ovulating but now my period is now late by, well today is the 24th you can do the math and no sign of starting at all. can i be pregnant. also take in the aspect that since i was 13 all doctors have told me i would never be able to get pregnant, but in december i was 2 monthes pregnant and had a misscarriage. but since my period started back up like clockwork until now