Miscarriage & HCG Levels

Hi 
I had a natural miscarriage on April 25th this year, and I have been having  my HCG blood levels drawn every week. The levels have gone down EXTREMELY slowly, but by last week, they were down to 21. Well, I got my results back from this weeks draw, and it's back up to 34. My doctor left me a message with this news also saying I might need to get a D & C if they stay this way-he ordered more blood work for Monday-has anyone had experience with thier levels going back up after decreasing and if so the reasons for it? Any info would be greatly appreciated!