Is this normal???

Gretchen will be sitting and playing everything fine and the next thing i know, if she holding something she will grap it and squeeze it till she starts shaking and she turns red, and grunting almost as if she is trying to poop, only she has been having her normal movements, so i dont think she is constipated... Its like she is turning into the incredible hulk... I googled it and they say it can happen when babies are teething but i have NEVER heard anyone say their babies are doing this...