I am 15 and have grown a lot in a short time. I have strecg marks on my inner thigh. Is this normal?

Stretch marks are scar tissue. It happens because you are low in vitamin E. Get "Pure Vitamin E Skin Beauty Oil" at the cosmetics counter. Be sure it says "Pure". You can also use capsules by poking a hole with a pin and squeezing the oil out. Vitamin E has to soak in, so put it on when you can leave the skin bare for a while.