Bleeding

I've had it off and on throughout (in 7w6d) and my doctor keeps saying that they have no way of knowing why it's happening and that I just shouldn't worry about it. But it's very alarming and I just had a good amount of blood a few minutes ago while going to the bathroom. For those moms out there whose doctors have diagnosed their bleeding, what have been the causes? I'm looking to switch to a different doctor that is more willing to explore what's going on with me. 😕