Why do I have this pain that starts out in my knee and travels up my thigh?

A little more specifically, this started very suddenly a month ago, when I was getting out of car. I took three steps and then had to stop and lean on a nearby car because the pain was so bad in my leg, radiating from just above the knee cap towards my inner thigh and traveling up to the top of my thigh. It happened quite often at first so I wrote this off as a sprain or pull but I still have it, only now it occurs when my leg is in elevated positions (strictly). Also the pain is not a sharp pain like you would get from a tear, pull or sprain. It is more like a radiating pain that throbs. I've had no injuries to this leg in a past or recently. I'm also 21 and a little overweight by about 15 pounds.