What Causes Throbbing Pain In The Upper Thighs?

Hello, I am a 29 year old female with a throbbing dull pain in my upper to middle thigh. I am a runner and had at one point thought it might be my hip flexor. I ran a half marathon last spring with the same pain and forced myself through the last few miles, since then i laid off running for awhile to let it heal. I have been up and running again for the last two months and the pain has come back. Im not sure if its serious or if i can continue to run, the pain goes away while running most times but i can feel it when walking and even sitting.