If I had unprotected sex and he ejaculated inside me a day after my period stopped... is it a certainty that I can't get pregnant??

You can get pregnant any time you have unprotected sex. Your likelihood the day after your period is low but not 0%.

I hope this was a one time, bad decision...

It's not a certainty but there is a chance. Protect yourself for the rest of your cycle and pray to god that you didn't conceive.

Unprotected sex is not good at any time. Been there, done that, and it is NOT worth the stress about worrying about being pregnant, and what people will think, and all that other stuff. Do yourself a favor and use a condom. I spent 3 weeks thinking I was pregnant, hoping praying that I wasn't. Protect yourself from heartbreak.