My left knee swells after exercise?

Internal derangement: Normal knees don't swell. You have probably torn a cartilage in your knee, called the meniscus. You need an MRI to evaluate. Don't ignore, as chronic tears can lead to arthritis.

Injury: Uncertain why but your evaluation will look at the stability of the knee to see if there is instability. The causes can also include: underlying arthritis syndromes, cartilage problems, hypermobility, internal knee injury. You need good knee muscle exercises, strengthening of the quadriceps muscles is part of this program. To clarify the swelling removal of the joint fluid for analysis.

Apply ice.: Ice the knee for 20 minutes at a time, 4-5 times per day. Take an over the counter anti-inflammatory medication, and see a non-surgical sports medicine doctor, to determine the source of the swelling.