My right knee hurts when I bend it. Trainer says it’s general muscle soreness. It’s better when straight. What could it be? It feels tight when bent.

You need an evaluate. This could be an internal knee problem you need an appropriate evaluation . Is it Muscle , is it a tendon is a bursa, is it joint, or is it Cartlidge. My biggest concern from what you tell me is it could be one of these - Cartledge would be the one I would be first thinking about.