Why every time i run my legs get swollen?

I am not sure: The reasons for edema (Fluid retention) are many, and underlying causes need to be evaluated and tested for, such as venous insufficiency to possible heart issues. Best wishes.

Edema: Get checked for fluid retention and circulation isues. I would advise abstaining until medically cleared,

Simple question: but complicated answer will follow. Without an exam my best guess is that you may be stressing your cardiovascular/circulatory system to the point that it can not keep the flow going in the right directions. You may need to take a break from running and visit your Doctor and perhaps run some heart tests to see if you are fit enough for strenuous aerobic exercising.