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There is no connection between sexual intercourse and knee pain
. Have you taken a knee X-ray/MRI to rule out arthritis
or joint problems like ACL tear or dislocation of patella or inflammation of tibia bone. There is a condition called Post orgasmic illness syndrome
in men who tend to feel sick and get muscle pains after ejaculation which could be due to hormonal disturbance. First contact an Orthopaedician to rule out knee problems. Best will be a Psychiatrist
to help you in this matter with proper diagnosis and management if no knee problem organically.