Thank you for your question. There are a number of possible causes of your symptom, ranging from simply trace mineral deficiencies like magnesium to more complicated inflammatory issues like a blood clot or a cyst behind the knee. I would recommend taking a break from strenuous exercise, increasing your mineral intake (either through the use of himalayan crystal salt or something like happy water or concentrace) and if there is no resolution in 24-48 hours to seek the opinion of your local GP so s/he can exclude the more nefarious causes. If it is a simple muscle tension then rest and massage will be of greatest benefit. If the minerals do work then great, but ask youself why you are mineral deficient in the first place.
Many thanks and warm regards