My last period was december 15, 2013. my bf and i have unprotected sex. took 4 pregnant test all came back negative. what could be the problem?

Still may be preg: Several things can change a regular period. If something has changed, it is good to consider pregnancy. Home pregnancy tests are pretty accurate, but no test is 100% accurate. Other things that can alter your period can be exercise, some medications, stress, weight changes. May be time to have a blood pregnancy test, or at least sit down and discuss things in detail with someone.