Pregnant maybe? (Implantation Bleeding) (Please help!)
I know this forum is mostly limited to women who are trying to conceive...I'm definitely not trying to conceive (yet), but I REALLY need advice. 
I had unprotected sex once exactly one week ago (Oct 6, we had forgotten to bring a condom and have used withdrawal method previously with positive results).
On Sunday night (early, I know!) I started feeling twingey cramps and pain. On Monday, I started bleeding rather lightly throughout the day. On Tuesday, I continued the moderate bleeding, cramps that resemble period cramps, and started feeling sensitive nipples.
Today, Wednesday (and exactly a week after unprotected sex), the bleeding got heavier and moderate, kind of like in my period. The blood was bright red.
Today, Wednesday, the bleeding got heavier and bright red. My nipples no longer feel sensitive, and I no longer feel twingey cramps, just symptoms that feel like period cramps as well as leg pain (as I always have during my period).
My period is due exactly a week after I started these symptoms (4 days from now). I know this is all happening a bit early to be considered, but I'm concerned. Is it implantation bleeding? Could I have had a very early chemical pregnancy? I took a Plan B two weeks ago, but in my prior experience with it, I did not experience a reaction like this...
I went to the OB-GYN, got a pregnancy test that tested negative, and they told me it was too early to determine anything. PLEASE HELP!