Need advice

So at the beginning of my pregnancy after my 7 week ultrasound my doctor changed my due date from Feb 6th to Feb 18th. But I have a planned Csection for Feb 12th. But now theyre telling me that my son is measuring quite large. I feel like they did wrong by changing my due date. Ive been thinking about it a lot. Me and my husbabd rarely have sex. So when we decided to try to conceive it was easy to isolate. Based on my last period of May 2nd, Id be due Feb 6th. Based off my conception dates of between 12th-15th of may Id be due Feb 4th-7th. Im pretty sure I concieved the 14th because at that time I wad tracking my ovulation with the strips and my peak was the 14th. And my fertile discharge also was earlier that morning. Based on that ovulation day, Id be due Feb 6th. Everything is pointing to Feb 6th. But when I got my ultrasound at 7 weeks they said baby was measuring smaller so they were pushing my due date to the 18th. But me and my husband only had sex those 4 days and did not have sex again until a week AFTER my postive pregnancy test. Then at my 20 week anatomy scan they told me he was measuring large for my gestational age. And he has stayed larger in the 2 scans ive had following that. Now im worried because if my due date is actually supposed to be Feb 6th, if they deliver me on Feb 12th ill be 41 weeks along. Has this happened to anyone or does anyone have advice or could tell me what im missing?