When i was little i had growing pains in my legs i'm 21 years old know and still get the pain in my legs what is this?

Lactic acid buildup: Especially after vigorous activity and when not well hydrated or not having had a good cool down period you could be having pain from lactic acid in the muscles. If this is the case, you would notice improvement with a warm bath to increase blood flow and massage. Some people experience this with normal activity, not vigorous exercise. Try increasing your fluid intake, warming up and cooling down after exercise or walking, taking a warm bath before the pain comes on. If the problem does not resolve with these measures, have your doctor check you to see if there is something different about the way your muscles handle energy requirements.