Measuring 2-3 weeks behind

LMP was 8/17. I thought I ovulated on 8/31 but that’s just a guess as usual cycle for me is 27-30 days. BFP came at 3 weeks 5 days on 9/11. I went in on 10/12 at exactly 8 weeks for first US. Empty gestation sac that only measured 5 weeks 3 days. No yolk sac or embryo visible. Took blood on 10/12, Hcg 26770. Went back 3 days later in 10/15, Hcg 30436. Dr seems somehow hopeful with rising Hcg that I just ovulated late even with only 10% rise in Hcg. Dr won’t see me until 10/26 (by LMP 10 weeks but after ultrasound would be 7 weeks 3 days) any success stories similar? My dr seems hopeful but I feel realistic that maybe this is blighted ovum. Numbers don’t add up to ovulating late with bfp test so early. First pregnancy since MC back in Nov 2020.