What Causes Pain And Purple Bruise In Knee Post Flight Journey?

I ve just flown from Atlanta, Georgia into Heathrow, London for a study abroad trip with my class. It was an eight-hour flight and I experienced pain in my left knee from sitting for the entire flight. In addition to that, my leg turned ice cold and I felt awful for the rest of the flight and could not seem to warm my feet or leg. Upon arriving at the airport the next day and walking around, I experienced little pain but have developed some sort of purple/pink cluster (veins, perhaps?) behind my left knee. It also hurts for me to sit for long periods of times - often my left foot will grow cold and ache. It also feels a bit stiff. I am twenty years old and a female college student. I am also on birth control. Thank you!