Negative blood test but no period and pregnancy symptoms?

my period is now 13 days late. I've taken three at home pregnancy tests and they were all negative. I went to the doctor for a blood test a couple of days ago and it too was negative. The doctor prescribed a pill called provera which she said would make my period come. However i'm scared to take it because i've heard of people that were pregnant and the only way they found out was thru a ultrasound, because all other tests were negative. i also knew someone who had a miscarriage from a false reading so she took the pill and lost her baby. For the past few weeks i've been nauseous, extremely fatigued, moody, hungry and thirsty all the time, peeing almost every 20 minutes, i've been so restless that i can't sleep, i'm having heartburn and strange little twinges and shooting pains in my stomach, i'm bloated and my hurt so bad that even taking a shower hurts. my have also gotten darker. my fiance and i have on a regular bases(sorry if tmi) so if i am pregnant it could have happened at any time. If i'm not pregnant then i'm wondering what else is going on, and if i am i don't want to risk taking these pills and losing my baby. Any advice would be greatly appreciated, and please be kind. I'm not crazy and imagining these things just because i want a baby. I know my body and i know when something isn't right. It's a gut feeling that as a woman you just know. thanks!