Hi
Welcome to the MedHelp forum!
Pain on either sides of the knee is usually related to meniscal tears, injuries to the collateral ligaments, and even to arthritis. A MRI of the knee joint would be diagnostic. Please consult an orthopedic specialist. Take care!
Since you are a personal trainer, it sounds like you have injured your knee.
Is this somehow more complicated than that - for example, you've been to several doctors and they can't figure out what's wrong?
Best wishes.