TMI question! My husband and I decided that we want to go for baby number 2. my son is 8 months old…

TMI question! My husband and I decided that we want to go for baby number 2. my son is 8 months old so we rather finish early because we want them to grow up together. we had unprotected sex yesterday but today I got my period For the first time since I had my son does that mean that I didn't get pregnant or is there still a possibility that I did yesterday. what you girls think?