Bleeding/spotting at 13w4d....please help (TMI)

On Sunday I had this gush of blood that came out, which then stopped. Since then I have been spotting on and off. Sometimes I will see couple of drops of blood on the panti-liner but most of the time it's when I wipe I see it on the TP. Recently it's mostly dark red/brown.
I did go see the doctor on Monday and he said baby looks fine, and he is not able to see what is causing it to bleed. 
I am extremely worried, and don't know what to do. Please let me know if anyone else had this happen.