Can i have sex after my period before my iud. if not why not?

Pregnancy risk: After your period is close to ovulation when you are most fertile. No iud in place means you are unprotected.

Have all you want: Pregnancy has been documented from unprotected sex on any day of a woman's cycle, including during menses. It is uncommon & would be more likely in those with irregular timed cycles, but there is risk just the same. You can have all the unprotected sex you want as long as you accept the risk of pregnancy yo have.