HCG levels

Hi everyone 
Got my faint positives for the last 2 days but today I started cramping a bit and light spotting. With my MC in November I panicked and went in for blood work. They are estimating me at 3 weeks and my levels are at 17.5 which she says is low but ok for how early I am. I am going back Wednesday and she says it should double. If not we may be looking at another MC, hoping and praying it's not.