Unknowingly could be preggers?

My husband and I went to have sex and we said we would use a condom I got one out and gave it to him he opened it but didn't put it on. I thought he put it on and he thought I knew he didn't. Anyways he got off inside me, and I didn't know till afterwards that he didn't have a condom.

I just ended my period a few days ago. We want to get pregnant next month but I'm not ready right now. Is there a very big chance I could get pregnant? Should I get the plan b pill or am I ok? Stressing out.