Heavy bleeding 12 weeks pregnant

Grace
I'm 12 weeks 6 days pregnant with twins. My most resent ultrasound was last week, everything looked great! I woke up this morning around 6 a.m. It felt as if I peed myself so I ran into the restroom and there was blood (bright red and watery) everywhere, soaking my shorts and all down my legs. I went to see my doctor as soon as they opened at 8 and I was still bleeding, just not as heavily. They scheduled an ultrasound for 9:40. Surprisingly to me, Both of the babies were moving and really active as soon as they started the ultrasound, we heard both heartbeats, and everything looked great. Nobody could tell me where the blood was coming from and I'm still very curious. Has this ever happened to anyone else?