Behind the knees

I've been running, to maintain/lose a little weight, but I suddenly like overnight have stretch marks behind my knees. Small ones but they are getting worse. I also have some on my inner thighs, they are red and super bumpy. 
I don't understand where they are coming from. I am 21 years old, I maintain my weight, I've never been pregnant. Should I go see my doctor about this? It just doesn't seem normal...