Could I be pregnant or am I just paranoid?

Okay, so for the last week and a half I have been nauseated (but no vomiting), headaches, and had a couple dizzy spells. I was supposed to start today, but haven't yet. Although I've had late periods before. Last night my lower back started cramping really bad, and it did as well this morning like I was going to start my period, but nothing has happen. I am not on birth control due to me being anemic and my doctor is afraid of blood clots...which makes no sense to me ata ll. So my husband and I use the withdrawal method, which I know isn't the best form of birth control. Anyway, just some thoughts on if I could be pregnant or not. I did take a pregnancy test a couple days ago, and it was negative, but I thought it may of been to early to tell.