Thank you for asking HCM
I have gone through your query.Your problem is most likely a haematoma
of knee.A krack or small fracture
can be suspected in that case so an x ray will be needed to rule out that.You can apply ice,put elastic bandage
on your knee and give rest.Consultation with an orthopedician is preffered .Hope this may help you.
Let me know if you have any further query.