What about a drop then rise in HCG levels...?

Okay.. Here's my story. I have been seeing a fertility doctor and this past cycle we did insemination. I had 2 good follicles and a 3rd that was close. We did insemination two days in a row. I got a very early faint positive pregnancy test on cycle day 26. On cycle day 28 I had a positive blood test with an HCG of 39. They repeated the blood test two days later (cycle day 30) and my level was 24.5. I was told I was going to miscarry and to watch for bleeding. They said they needed to follow my numbers back down. I went in 5 days later (cycle day 35) and my HCG level was a 4. I was told I would start my period within a week. Well things got busy and I forgot about it. I have had consistent cramping feelings for the past 3 weeks or so (to the point where I would wear a pad thinking my period was going to start at any time) but no other symptoms. I finally called the doctors office telling them that I hadn't had a period and they wanted to check my HCG again. Well... 18 days later (cycle day 53) my HCG level was 638!!!!!! What the heck does that mean?? I did look back and from the date that my HCG was a 4 to the date that it was 638 was 18 days between and then the doubling rate from the 4 to 638 was every 2.5 days which is within normal. Is is possible that I had the vanishing twin syndrome? or maybe released the third follicle later and got pregnant on my own? Anyone else have successful stories of drop and then rise of HCG? I do go in on Monday for another blood draw and ultrasound to see if they see anything. I had totally accepted the miscarriage and am scared to get excited but what does this mean?? Technically I am pregnant?!?!