What are the chances of catching an std if i have sex with a girl on her period, even if i used protection?

Depends: Consider the exposure, menses or not a real exposure. Some stds are not totally prevented by condom use and if they usually are it is only because you used it corrrectly. Gloving up before any contact is important as is regloving if you go another round.Some germs like herpes and hpv can be active on surfaces you wil come into contach with that are not covered by the condom.