My thigh hurts and I can't stretch that leg out fully. Why could that be?

[VivienN]

Since a couple of years ago, I am unable to fully stretch out my left leg. It doesn't cause a problem during walking, but for example if I'm sitting with a straight back, and try to raise my legs, my left leg is unable to fully stretch out, and feel pain in the back part of the thigh, ending at the popliteal. Same if I'm standing and trying to bend down without bending my legs. The thigh also feels swollen compared to the other.

I also noticed a little ball in the thigh a couple of weeks ago, just at the area where the pain starts at. It's around 1 cm, and although it doesn't really protrude from the skin, at closer inspection the skin above it is slighty darker/greener than my skincolour.

It's a problem because I have started to dance again, and it stops me from doing certain moves.

Thank you for your attention.

Sincerely,
V. N.