Feeling light headed while working out?

I'm 15 year old. When I weight lift in the gym I do intense workouts to build muscle. After I get done with an excerzize I feel light- headed and my muscle literally feels light as a feather (especially my legs after I do curls). I know about this thing called "runner's high" where you feel light- headed after running a lot and I think its that sort of thing. Except after the exercise my muscles feel so weak that I can barely use them- after I do curls my legs wobble when I try to run- but its back to normal after a minute or two. I feel faint, but not like I'm actually going to faint, it feels a little good actually. Is this normal? Is it bad for me at all?