Spotting....

I am 8 weeks pregnant. I had sex the other morning, went an cleane up afterwards and there was no blood. I went to the petco to run errands an came home. I had to pee so I did an when I wiped there was a lot of blood! My doctor sent me to the ER. I had just had an ultrasound the previous day via tummy. They heard a healthy heartbeat of 162 beats per minute. Everything looked good. My bleeding lasted around 20-30 minutes if that. At the ER they took blood, urine, and did an ultrasound to find a healthy baby measuring one week ahead of schedule at 8 weeks 6 days and heartbeat of 142 beats per minute. The spotting quit yesterday and happened once again today brown and so ridiculously light. What could this be? I haven't had much cramping or pain at all. Could this be stretching, miscarry, just the sex....what could be going on? Anybody have a similar story??? Help please! I'm a first time mommy!