My friends dog?????????

my friends (twins) have a dog who is a pom-chi. pomeranian chihuahua mix. and he has been acting weird lately but i don't know whats wrong. his stomach is hard, he shivered~he stopped by now and sometimes you'd pick him up a bit on the stomach and hed do a yelp?? also, they said he pooped lately and its not affecting him, only if we touch his stomach. because today he was playing with my chihuahua roxy. um if this helps, they're both fixed and he acted like that after they gave him a bath. hes going to the vet friday though. well also hes usually really excited when i come bc he pees and wont stop moving and kissing but yesterday he was all calm..not at al like him! well id like to know so i can tell them. hes a year old and i don't want anything bad happening. well thanks!!!! :-)