bleeding @ 12 weeks

So yesterday I woke up to spotting I freaked out and went to the ER after 4 hours, an ultrasound, some blood work, and urine samples i was told my babies were good and everything looked good no reason on to why i was bleeding. Today I began bleeding heavier than yesterday. I was seen by my ob and her to said she couldn't give me a reason. Heartbeats are good and babies are moving. Placenta looks good and no polyps. Has this happened to anyone?