Help please

Hi all. I need some advice, please don't judge me .

My boyfriend and I had been trying for a baby for about 14 months, I have polycystic ovaries so really irregular periods ( sometimes months without ). In march i had a

Family bereavement and my boyfriend and I ended up splitting up for a couple weeks. On may 19th I slept with someone else just the once ( whilst we wasn't together ). On June 14th I found out I was pregnant but had no idea how far due to irregular periods. My first scan was done just before ten weeks and it gave me an EDD of 25th Jan. My next scan at 12 weeks 5, showed my EDD as 21st Jan.

According to online calculators 

That would make my conception date April 29th. My boyfriend and I got back

Together.

Is there any chance these scans dates

Could be 3 weeks out?? It's literally ruined my whole

Pregnancy worrying. The baby's always been measure slightly bigger , so never 3 weeks under.

I really need some help as I can't stop thinking about it.

Thanks x