What is wrong with my dog ?

I have a papilon dog he having trouble with his tail it wont stay curled up and when walking him he having trouble trying to poo this has been going on for about 2 or 3 month now had him at vet for xrays and nothing showing up he is 10 yrs old im worried about him and with his tail down he gets dirty at the back and when i try to clean him he yelps and tries to run away any help would be grateful thanks