What’s going on? Please HELP! Worrisome lady here!

I went to get my first ultrasound and from what my midwife told me based on my period I’m supposedly 13 weeks. Even this app says so, but based on my ultrasound my baby is 9 weeks. My period is super wacky, like 36-45 day cycle instead of the normal 28 days. I got my period Nov 7, took a pregnancy test on Dec 22 - was negative, did one a week later Dec 28 - was positive. It took longer than 2 weeks to get a positive result. Should I worry about my baby’s fetal growth or am I a pregnant lady on a whole different spectrum of how big my baby should be due to my wacky cycle?