The Psoas is one of the most important hip flexor muscles that you have.  It’s the small, mostly verticle muscle that runs right along your hip on either side.  The reason it is so important is that you use it whenever you lift your leg or rotate your leg externally, which I’m sure you realize now that you do a lot more than you thought you did now that your experiencing psoas muscle pain.
Common causes of muscle pain include:
--Dystonia
--Fibromyalgia
--Hypothyroidism
--Influenza
--Chronic exertional compartment syndrome
--Chronic fatigue syndrome
--Claudication
--Lyme disease
--Medications, especially statins
--Muscle strain or rupture
--Myofascial pain syndrome
--Polymyalgia rheumatica
--Polymyositis
--Porphyria
--Delayed-onset muscle soreness
--Dermatomyositis
--Lupus