18 not pregnant 3 missed periods?? please help me?

On February 6/2011 I randomly did a pregnancy test it came up very lightly as positive.February 7/2011 I went to the doctors because I was bleeding and they said I miscarried and my HCG level was 4. Today I saw a really nice and smart doctor in emerge and he advised me it was IMPOSSIBLE for me to have been pregnant and with in hours miscarried adn my hcg level dropped to 4. He said there is no way my hormone level could have dropped that QUICK. He is now telling me he can't help me in emergency I need constant guidance from my family doctor to figure this out. He said it could be so many different things and he will only mention the positive. He said mother nature may fix this on her own but he can't diagnose. I have 2 children ages 2 and 4. Im 5'5 and 155 lbs and ive been like that for 2 years now. NEVER have I missed a period when not pregnant. Had a blood test today and again negative my urine is also completely normal. I had an IUD until last July It was removed because it caused me to bleed alot randomly. I never got an answer to why I was bleeding on it. February8/2011 I had an ultrasound everything looked good. My activity level has not changed and I have been far from stressed. I eat like I have always eaten which is pretty healthy. I have a lil pain during intercourse but only when it goes up to far. I am really stumped as to what this can be. I have not had a period since feb 7/2011 and now its been over 13 weeks i think it might even be 14 now. My mom and grandma keep telling me cancer. My mom stopped getting her period and next thing she knew she had cervical cancer or something like that. They keep saying cancer or hemorrhaging. I have avoided going to a dr about this until today when they mentioned that. My nana lives way across the country and is praying for me tomorrow but I just want to know if this CAN be a serious issue and what kind of things can it be. I haven't really had any other symptoms odd time a lil cramp here and there and that's about it. Please help me? All ideas please. Anyone with similar experiences?