What Causes A Knot In The Leg After Pulling Of Muscles?

I was running the other day and i could feel my leg muscles pulling but didn't think anything about it but then in this one spot my leg started burning real bad, when i got done running i had a huge knot that popped up in that spot and it doesn't hurt unless i start running again and it goes away when i sit down or don't stand on that leg. Any idea on what that could be?