Why am I late?

Had protected intercourse with a spermicidal/sponge and condom on the 12th, 18th and 19th of March. My last menstration was the 22nd of February. I'm normally regular. Took two tests, one last week and one this week and they are negative. I'm freaking out, I'm stressing out not only about this but about bills and school. I haven't visited my doctor yet bc I'm very busy. Why am I so late!!?? I exercise five days a week, I've been having cramps, I'm stressed out, I gained 2lbs in the past week (I do lift weights) but this is scary! No period yet!