Help..

So when I found out I was pregnant with my son I had 2-3 periods but when I took a home test it was positive and I went o the doctor thinking I was just a few weeks.. I ended up being 12weeks and 3days.. But he is almost two and I'm kind of worried it might happen like that again.. I had sex with a guy like the 18th of December but we didn't use protection an I was fertile.. So idk what to do