With RI my jaw starts to hurt after a while, is this right?

No, some people tend to move their jaw forward when doing RI (The danger is that it is an unconscious thing. Once you are aware of what is causing the pain, it is easy to overcome). This is not necessary.You can easily get rid of this by moving your jaw when playing RI and bringing it back into a more relaxed position. There are people that benefit from bringing the jaw forward a bit (not only in RI) for a better balance in mpc pressure on both top and bottom lip. But if your jaw starts to hurt you are definitely overdoing it.