Um help

Went to my boyfriends house last night and I started my period yesterday morning. Had a tampon in and we messed around a bit and he ended up getting precum in my and then came around my vagina and got someone in me. But still had a tampon in. Ran to Walgreens and took plan b an hour later. I read the risk of getting pregnant on your first period day is virtually zero...should I worry even though I took plan b?