Baby's due date changed?

According to my last period I should be 15 weeks, 4 days. Ultrasound is showing baby at 13 weeks, 4 days. My period was July 20-23 Then I was due Agust 17 but my period came Agust 25th again for 3-4 days So in September I was ovulating around the 5Th, we had sex during the ovulation days. Period was due semptber 22nd, never came. By October I found out I was preggo. I'm confuse as to why the baby is measuring smaller. I nevet had any problems with my other 3 kids. Can someone explain it to me? Feeling scared. Thank you!