Medial knee pain but no swelling, im a swimmer that swims breaststroke. ?

Many possibiliities : It could be related to various structures- meniscus, medial collateral ligament, tendon problem (semimembranosus hamstring tendon, pes anserine tendons ) or bursitis or referred pain from somewhere else. Should be examined by a knowledgable medical provider and possibly an MRI or musculoskel ultrasound if it persists for awhile.

Variable: Strain or synovitis. Rest, ice, compress, elevate and nsaids.