I had sex for the first time yesterday on the first day of my period. it's now the second day and my period and cramps are heavy and painful. normal?

I think that could: be considered normal even if it is a little unusual for you. One couldn't know if the reason for the cramps has anything to do with sex the day before.... probably not. It is probably just a coincidence; I would't worry about it. I presume you chose the first day of your period to avoid pregnancy, but I suggest even more reliable contraception. Need condom, too, for STI prevention. Stay well:)