What Causes Pulsating Sensation On Back Of Knee?

Hi, my daughter has this weird thing going on with her leg and I am wondering if I should take her to the doctor? Her leg occassionally above the back part of the knee starts to pulsate very hard, you can see it and probably lasts just under 5 minutes she complains it hurts a little bit but then it goes away and their is no raised bump or anything. What could this be or should I do? She is 9 yrs old 4 foot 1 59 pounds