wrong dates?

Went to EPU for my scan yesterday and was told that I'm not 10 weeks like I thought. I'm actually 5 weeks. They said it was too early to see/hear a heartbeat. I have an appointment to return to EPU for another scan on Friday. I got told that if the baby hasn't progressed then I am definitely having a miscarriage. I just don't get how my dates could be wrong. My last period was around about the 16th August. I had been on the pill for 14 years and wonder if that could of made me ovulate later than I thought? I'm currently still cramping and bleeding. It started Monday night and I thought it would be over by now. I don't think it is a very good sign although I am trying to stay positive. Has anyone else been through this before?