What Causes Fever With Pain In Shoulder Blade, Back And Upper Abdomen?

I had a 103.8 fever 4 days ago with shoulder blade and back pain. Since, the fever goes away during the day but comes back at night, not nearly as high but comes back. Also, I have this extreme intermittent, sharp, stabbing, muscle spasm like pain in my upper abdomen. It s been 4 days of this, what could be the problem?