I noticed I was bleeding after sex very very light but it made me worried I only had sex with an one person so what could be wrong

Light bleeding after sex isn't unusual - tiny tears of delicate tissue, or coincidence with ovulation spotting, or getting your period a little early. When you say you only had sex with one person, do you mean your very first time, or that you are worried about disease? Bleeding isn't a sign of an STD. But you need regular GYN checkups anyway, so you can be tested for them while having an exam and pap smear, etc.

No it's not my first time and this was after my period went off