Bleeding 14-16 weeks

I started bleeding red 2 weeks ago which then turned to brown. I then had 24/36 hours without bleeding but then it started again, dark red which turned to brown and currently I am still bleeding, had 3 ultrasounds and can't find the cause of bleeding but baby is doing fine (at the minute) I have some pains and cramps in the past couple of weeks, but nothing too serious. Has anyone heard of anything like this or experience unexplained bleeding? Im worried sick!