Came home from school and found my dog with tail tucked between back legs, whats wrong?

I tried calling him, but he didn't come to me which is weird because he always does. I found him laying down. when i petted him, he followed me but i noticed that he was walking strange. and his tail wasn't wagging, it was tucked between his legs. i thought maybe his tail got stuck to his anus because he gets poop caught in his hair a lot so when i tried to gently move his tail, he cried out in pain. he didnt bark at my sister's boyfriend (who he hates A LOT) and he kept shivering. i brought him to the vet immediately and they did x-rays on him. they found no bones were broken and they were ok. he told me something about poop but i couldnt understand a lot because he had a strong accent. But then i gave it some thought, and i dont think he's constipated because he pooped yesterday at 1am, and he was still eating and drinking. Last time i saw him healthy was 12pm today, he was PERFECTLY healthy. then left for school and came home 2 hours later and found him like this. so i think he might have been abused by my sister's boyfriend, because my sister's bf tends to messed with him when im not around... he really hates the dog. The doctor gave me pain medication. What else can I do? and what else could be wrong with my dog?