Iliopsoas syndrome, which is also called psoas syndrome or iliopsoas tendonitis, occurs when the iliopsoas muscles are injured. Lower back pain is the most common symptom; however, pain can also occur in the hip, thigh, or leg. The iliopsoas bursa, which is a fluid-filled sac located on the inside of the hip that reduces rubbing and friction, is also likely to become inflamed due to the proximity of the two structures. When this happens, the inflamed bursae will make it difficult to move.
2016 — More editing, more! Added some better information about pain being a poor indicator, and the role of myofascial trigger points. This article has become extremely busy in the last couple months — about 4,000 readers per day, as described here — so I am really polishing it and making sure that it’s the best possible answer to people’s fears about back pain.
Some of these red flags are much less red than others, especially depending on the circumstances. For instance, “weight loss” is common and often the sign of successful diet! (Well, at least temporarily successful, anyway. 😃) Obviously, if you know of a harmless reason why you have a red flag symptom, it isn’t really a red flag (duh!). But every single actual red flag — in combination with severe low back pain that’s been going on for several weeks — is definitely a good reason to get yourself checked out.