What Causes Low HCG Level During Pregnancy?

I recently learned I was pregnant by my doctor...had a hcg test my number was 35 my second test my number was 48...the doc first suspected a miscarriage now she knows something is goin on hopefully not a eptopic pregnancy..???im worried out of my mind...I'm having some cramping not bad just noticeable...read online that as long as I'm not bleeding or the pain isn't severe it's ok???please help thanks