It's hard to say which causes which. But I do know that the mind and body are inter-related in ways most doctors are just now beginning to accept. Some illnesses don't show up immediately through tests, such as Multiple Sclerosis. Rather, illnesses like these are most often diagnosed as the physical symptoms wax and wane or as they intensify. Lots of physical illnesses do cause severe depression.

That said, depression can cause changes within the brain and body. Depression is known to affect memory as well as bring out more awareness to discomfort that ones body is experiencing. And if you aren't exercising regularly, that also affects bm regularity as well as muscular aches.

Personally, I aim towards knocking out all of the possible physical causes before moving onto psychological. Just due to my experience with my epilepsy being misinterpreted and diagnosed as simply panic disorder. Reality was that I had epilepsy, which went untreated and it brought on panic disorder. That's my experience.

Very best wishes to you!