Hit right knee very hard on corner of counter. swollen and red with trouble moving it. is their a risk of a blood clot?

Blood clot. : Unlikely symptoms related to blood clot. More likely an effusion of the knee joint from trauma. If it continues to be uncomfortable see an orthopedic surgeon and they may recommend aspirating the fluid.

Not your main prob.: Blood clots would change circulation and you'd be expected to have a problem with that from the knee all the way down. Alternatively, fractures, or bruises or tendon injuries would cause localized swelling. Heat and anti-inflammatories are often helpful.

Possibly.: Prolonged resting the knee and being immobile can increase the risk of a blood clot. So make sure you are exercising your toes, ankle and do breathing exercises and keep well hydrated. if being immobilized for more than 3-5 days, your Doc should start you on some blood thinners for a few weeks. See your Doc..

Very low risk: I think the nature of your injury is more likely to cause a patella contusion, slight fracture or a prepatella bursitis The more likely reason you are having trouble moving it is from the pain effusion or a possible fracture