Wow sweetie, what a scary experience for you and your DP!
It's also a shame the Dr couldn't give you an answer as to what it may have been. I had some bleeding early on in this pregnancy after dtd, not to the same extent as yours though and my midwife told me it was due to the blood vessels inside me being more full now or something like that.
I certainly hope the bleeding stops very soon and your little bundle continues to be healthy & active