Question for moms pregnant RIGHT AFTER a miscarriage!

Amy
My best friend had a MC about 5-6 weeks ago. She has been going to the dr every week for blood draws to make sure her HCG levels were going down. Last week she was at -1 and now this week she's at 89!!! 
Is it possible?!?! She's going to get a pregnancy test on her lunch hour. But doesn't want to get her hopes up. Could it be ANYTHING ELSE besides a rainbow baby?? Would numbers jump like that for any other reason??
Thanks in advance!