just stressed or am I actually pregnant?

hello! my periods are frequently all over the place but I have not had my period in 2 months! however, I am always consistently stressed. (I'm an excessive worrier and stresser, it's a bad habit I can't break.) I have taken 4 different pregnancy tests that all say no. is it really just stress, can the tests be wrong? I also haven't had sex since October. because my fiancé is stationed overseas. I did have a period in November but it wasn't normal. help!!