Panicking !! Bleeding at 8 weeks

So I'm 8 weeks and 5 days pregnant. I woke up this morning with dark brown blood (small amount). There's a little bit of bright red blood when I go to the bathroom. 
I called my OB and she said it was because of having sex last night and to call if it gets worse. This same thing happened to my at 4 weeks. Some spotting. I just don't know what's wrong and haven't even been to my first appt. I had a blood test and at 4 weeks and showed hcg at 25,000.
Any advice? Has this happened to anyone else?