Bright red blood followed by dark brown.

Mallory
I am 3 weeks and 6 days, yesterday when I went to pee I had bright red blood as if I was starting my period and now everytime I go to the washroom & wipe I'm getting some brown. Only when I pee, it's not getting onto a panty liner.
I went to the hospital and they sent me home without any answers. My levels were 170, which were hard to read anything from that because they could be very low anyways because I am not very far along. Or the other option they could be low because they've stopped rising and my body is preparing for a miscarriage. 
Just looking for some hope that it may not be the worst case scenario here. Go back Monday for an ultrasound but the dr warned me it may just cause even more questions because they may not be able to see anything this soon. 
I only have very light cramping almost as if I am going to start my period. Does a miscarriage start like this?