PAIN OUTSIDE OF LEG JUST BELOW KNEE

This is my first post so im sorry if this appears in a weird place like on someone else thread but couldn’t find the new thread button.

I have recently aquired a pain in my leg that normally comes on after activity such as skating or kite landboarding. It all happended when I was kite landboarding on the beach and after I finished (without any nocks or falls) I notice that I had a pain in my left leg on the outside just below the knee. This resulted in me not being able to walk that night. Any rotation from side to side (not up and down) caused bad pain in the same place. This gradually faded away until I went on holiday and was involved in a lot of walking. After walking around for a bit the pain started to come on gradually and get worse and worse until I had to stop. One day a massive bruse appeared across the front of my shin. I still have the pains (not as much, but they are there)..What do I have? Please help.