Yawing is aggressive body language in guinea pigs it means "look at how big my teeth are, don't mess with me". They don't yawn when they are tired like us, they yawn as a warning before they bite.
It is sometimes accompanied by teeth chattering, which means "back off", it sounds like loud teeth grinding. The stretching and laying down is also "I'm not scared of you", so accompanied by aggressive body language it means they WILL bite if you keep bothering them.
They don't generally like their bottoms scratched, usually when something is touching their back/bottom it is another guinea pig mounting them. If a guinea pig is a more dominant personality they will never respond well to their back/bottom being touched as they really don't appreciate being mounted.