Period on time, 10 days later positive test

Cindy • My DH and I have 1 child together and have 4 all together. I have 3 from a previous marriage. We are currently trying for our last baby. We've been married since 05/14/16 and together since 02/10
October 13th I had my normal cycle. Lasted the normal amount of days and was normal heaviness. Then 10 days later hubby and I had sex and I started spotting bright red, not a lot but it was odd. Took several hpt and they ALL came up positive. Called my Dr and he ordered a quantitative hcg test. First levels were 82, 48 hours later they dropped to 39. So, not a viable pregnancy. Has anyone else had this happen to them? My ob is baffled by it. He does not understand how I could get a positive on cycle day 38 and have my normal period on cycle day 28. Also, I was taking femara and progesterone. Due to not ovulating properly and low progesterone levels. When I started my period I stopped the progesterone, as I have been told to do so. I still haven't had any bleeding, since the spotting, no cramps, nothing. Have no idea what the hell my body is doing.