I have flat feet, but only my right foot hurts a lot. the pain is now noticeable on the left side if my right knee. is there a way to stop the pain?

Flat feet: See a podiatrist who can prescribe for you custom-made orthotics to address your flatfoot condition that is now affecting your left knee.

Foot pain: Need more info: what part of foot hurts? Back of heel? Front of heel? Arch of foot? Ball of foot? Foot pain may b causing change in gait that's putting more stress on knee. See pcp or podiatrist for evaluation & treatment. May b plantar fasciitis (heel spur), morton's neuroma, etc. Correcting foot problem may also solve knee problem. Nsaid for pain & inflammation. Rest, elevation, toe stretching.