Heavy bleeding at 12 weeks

Colleen
Early Friday morning I woke up from heavy bleeding. I went to restroom and blood filled the toilet. I was passing huge blood lots that at the time I thought could have been baby. I wasn't in pain though. I went to drs office forst thing and they did an ultrasound and baby was perfect. Bleeding was not coming from baby at all. I did have my Mirena in for 3.5 years and got pregnant as soon as I had it removed. Does anyone think this could be a factor? I'm only asking on here because my midwife couldn't tell me why or what caused the bleeding. She didn't seemed too concerned since baby was great. Any experience with something like this? Thanks