I thought it might be a muscle strain too, but worried about offering an opinion that could also have been just as wrong. I have to take my dog into to check his legs all the time due to his medical conditions. I always come home with " well he is fine but so in so" but at least he went to the vet.
You and your little baby are in my prayers. It is probably just a strain but better to know for sure- especially with such a tiny dog. I am sure the screaming scared you to death. My guys are drama queens too. Its a frightening noise!
Keeps us updated- and prayers and hugs to all...!!!!