hCG levels

Julie
This is my 6th pregnancy, the last three were miscarriages I lost early on. I'm sooooo nervous that it's going to happen again. 
I got my BFP on monday, day 34 of my cycle. My hCG the next day was 651, two days later on Thursday it quadrupled to 2500! I'm not sure if this is good or bad! They are still on the low end...I'm getting another level on Tuesday (Monday is a holiday) and will have my u/s next week too. 
Anyone have a similar experience with levels rising quicky like that?