How long have you had her? Is it possible she could be gravid from before you purchased her? Because she looks very plump and that is a pretty extreme taper on her tail. I don't know for sure but it seems like her "sausage butt" may be due to eggs.
The only other thing I can think of is constipation, but I don't believe that's a common problem for BPs. I think it's pretty normal for them to go several weeks at a time w/o defecating.