Scan date wrong ??

Ok so this is going be a long post rewind to Oct 2021 I ended a relationship, I suffered side aches and ended up in hospital on Nov 4th had a bleed for 1 day and became nauseous that month and noticed my boobs were big as I was always flat chested due to a diagnosis of anorexia nervosa I then met a boy I went on a date a few times no intercourse on the 6th of december spent the nigh wit him let's just say the intimate stage just was not happening for us so we didn't do it, fast forward to Jan 23rd i noticed I had not had a period since the end of September which is normal for me as I have pcos so always irregular and ovalate late, I had a scan done and it was identical twins one twin measuring a week smaller, and the dating scan was saying I was 7 weeks pregnant which I could not understand, I went back for another scan 2 weeks later and I was then 11 weeks by scan date but unfortunately one twin had passed, so going by dates I conceived end of November start of December but I could be positive it was from my previous relationship that ended in October but doctors say no because there was a 3 and half week difference, I asked could babies have been small due to my eating disorder they couldn't answer so all was looking like the bot I had been seen start of December which was impossible as we didn't actually have intercourse, I stopped thinking about it and got on with he pregancy fast forward to 35 weeks I went I to slow labour and hospital stopped it got injections for lungs was told I would have a small baby due to eating disorder the at 39 weeks baby was not moving, had a scan and he had no fluid left and placenta had stopped working I was induced that night and baby came out like an over due baby long nails wriggly skin, im so confused because he was not small at all he had absolutely no room left , and to add he looks nothing like the boy I was seen in December he is dark like my previous partner ,,,, im so confused he is 5 weeks old now but eats and looks like a 8 or 9 week old my gp says very advanced , my question is could it be possible the scan dates were wrong as I had pregnancy signs and symptoms before I dated the boy in late November early December, which would point at my previous partner but dates point at boy I was seen but I can swear we did not have intercourse as it was not happening for us that night

Anyone any feedback or similar situation no nasty comments please