Sex for the first time

Postpartum I’m now 4 months and just has sex and I know it’s suppose to hurt the first time and trust me it did it wasn’t very comfortable now my vag feels sore and irritated is that normal? Anything else help? I want to try lube since hormones dried it up down there lol Also I’m curious I know your the most fertile after you have a baby and can get pregnant like that but I was always curious how this worked say I’m suppose to start my period in 3 days and I have sex now could you miss your period if you got pregnant that time you had sex or would you still have your period but then get pregnant I was always confused how that workers