Hi, My Lmp Was 3/14. I Am Absolutely Sure Of

Hi, my lmp was 3/14. I am absolutely sure of that but no idea when I would have ovulated. On 4/13 which should be day 31, I took a pregnancy test and it was positive, an hour later I had bright red blood but not heavy. Enough to change a thin panty liner every 4 hours and even then, it wasn't soaked, just covered enough to change it for hygiene sake (the bright red bleeding lasted 24 hours, followed by 24 hours of light pink and then 24 more hours of very light brownish spotting that did not even come off on panty liner). I got in to see my obgyn in the afternoon and her urine test came up negative. She checked and said my cervix was still closed and sent me to test for hcg. It came back as a 15. I took one again 65 hours later on day 34 and it came back as 36. My doctor said she is very concerned and that she believes the only way this is a viable pregnancy is if I am incorrect about my lmp but unfortunately, I am positive about it. Could I have just ovulated late in my cycle? Have you seen numbers this low this late in pregnancy? Is there anything I can be doing? We weren't trying, but we were so excited :(