I Am A Fit And Healthy 27 Year Old Woman.

I am a fit and healthy 27 year old woman. I am not over weight and I exercise regularly. I went snowboarding a few weeks ago. I didn't sustain any injuries. Since I have been back I'm having great trouble getting up from a crouching position back to a standing position. My knees just haven't got the strength to lift me but I never had these types of problems before the holiday. I came back some time ago now do I'm starting to worry it isn't just the snowboarding not holiday? Why am I having such trouble? When will it get better?