What Causes My Erection To Weaken During Intercourse, And How Can I Treat This Issue?

Hi, I am 90kg weight with 175 cm height Recently i am Feeling Problem of weaker Penis Erection during Intercourse . My Penis feels erect and Strong before Incerting in Vagina While During Intercourse within Few. Minutes It looses it s erection And Becomes Normal What is this Kind of Dysfunction And how to Treat This Problem .