Painful Buised vein from inner upper thigh to back of knee?

I was pregant with my 5th child when I got a popped out, painful vein on my upper inner thigh. Then shortly after delivery it flatted out and ran to the back of my leg almost to my knee. It's very painful when am active, walking, basically when I'm standing. When I am on my feet for over 30 min,.the pain runs down to the bttom of my foot, I then feel as if a vein popps in and out of the bottom of my foot. The pain does ease when at rest or with a support hose.  A sonogram was done to check for a blot clott, but not a thing was found. I'm only 25yrs old, 5'2, about 110lbs, and have always been healthy. I would appreciate it if someone may help me know what may be wrong with my leg.

Thank-you-Lorena