Nervous about HCG levels

Crystal
As of last period am 6 weeks and 2 days. On 5/27 my level was 135 and on 6/5 it was 5721 which is great but as of today 6/8 it is 8637 so it hasn't even doubled in 3 days... Should I be concerned? Also testing on Friday and today were done at two different places, I don't know if that would make a difference.