Ok. here's my dilema

i stopped taking my birth control pill for about about 2 weeks and started taking a weight loss supplement. I than decided to start taking my birth control pills again at the same time taking the diet pills. my last period was on January 17 of 2007, it is now march 13 and i haven't gotten my period. I bought a home pregnancy test and it came out negative. However, i get cramps, headaches and an occasional nausea ( not very often) due to the fact that i have this pregnancy doubt in my head i've stopped taking the birth control pills again.

has this happened to anyone else and could anyone give me any advice?