I'm guessing it may be a fibro flare - sounds like the problems I had from injections in the past. The muscles can go into spasm, and the muscle just keeps on tightening. Warmth with gentle massage, and a muscle relaxer like Valium can help. As for Septic Arthritis - I'm highly doubtful (I'm not a doc so would never say anything as a certainty), my son suffered from Septic Arthritis as a baby and it was an infection that got into his bloodstream and ate away his hipbone - a very nasty viral disease.
If you're concerned I suggest you contact your regular doctor.
Hope you're feeling better soon.