Not to scare you or anything ... Just want to give you my experience.
They're usually able to hear the heartbeat at 10 weeks pretty good. Sometimes they're not able to find it, it's very common.
I had to go to the er with my second pregnancy because I was cramping, fainting and lightly spotting. They couldn't find a heartbeat, either (at 8 weeks) and they decided that they were going to schedule an ultrasound the following week. I went in, happy go-lucky, feeling pretty good and they told me that the heart wasn't beating and I was in the process of miscarrying. So, I ended up having a d & c a few days later.
With my next pregnancy, I was a nervous-wreck. I always thought that I may be miscarrying and I was just a total mess! I had a normal pregnancy (aside from all the hell I was putting myself through) and gave birth to a healthy boy :-)
i wouldn't worry too much about it until the dr's give you a reason to. You may want to ask for an ultrasound, though, just to be on the safe side. Good luck!