What Causes Delay In Menstruation Other Than Pregnancy?

I was on birthcontrol pills for three to four weeks in November but then got off because they made me sick in December my period came a week and 1 day early and Janurary it came a day early now its Febraury it feels like i have pregnacy symptoms and i hadnt got my period yet me an my bf and i had protected sex on the 15th of January I've taken 4 pregnancy test including 2 of them after missed period whats going on!!