Bleeding at 17 weeks. . .

So I am 17 weeks pregnant and yesterday ended up in he hospital because I started to bleed. It started the evening before with just old blood spotting and so I went to see my OB who did an ultrasound and everything seemed fine. Only 4 hours later I had a HUGE amount of red, new blood and my dr told me to go to the ER. They did more tests and ultrasounds and said everything is totally fine with the baby and the placenta and couldn't figure out where the blood came from. So far today I haven't had any more new or old blood which is great. I wanted to know if anyone has had any experience with bleeding during pregnancy. X