I have had that same problem, pain in my hip/lower back area at the s-i joint (if you look in a mirror, it's about at that spot where there's a little line or dimple that most people have). After much confusion, failed tests, (well all but a slight postive in a fana, which is a rhematoid test), I was finally sent to a rhemuatologist who diagnosised me with anklosing spondylitisis or as (i'm sure i'm misspelling it). It's a form of rheumatoid arthrisis and is more prevalent in men than women (lucky me) and also common in people from 15-30ish. With this disease, lack of activity (i.E. Sleeping or sitting for long periods of time) causes the joints to to inflame, causing stiffness and discomfort in the morning and etc. If a person does not seek treatment, which can simply be anti-inflammatory medications such as celebrex, it is highly possible for the effected joints and future joints to fuse together, leaving nothing but solid bone where a joint used to be.