I have pain in my left side groin area its OK when I rest and sit but when I start walking the pain comes back and it hurts to keep walking

I have pain in my left side groin its when I walk and it hurts but its OK when I rest once I start walking pain comes back,and I don't remember straining or pulling a muscle had it for a few days comes and goes but today it really hurts when I walk for to long resting helps but I don't no what it is and why