If my partner ejaculated in me and I began bleeding a little and started my period is there still a chance of pregnancy

Are you on birth control? Are you using it correctly? If the answer is yes then you won’t get pregnant. Use condoms if you don’t trust your birth control.

We’ve decided to try for a baby so not on birth control

You’re best to ask on pregnancy/TTC/parenting forum or subreddit instead of here, most people on here do not have children and all are trying to avoid pregnancy 😅