I strained my hip flexor a few months back. It seemed really serious, as I could barely flex my leg at all, let alone put any weight on it for 3-days. I was certain it was a femoral neck stress fracture (symptoms can be quite similar, and w/my history, etc.). After 3-days of being nearly bed ridden, I had an MRI to rule out the SF. It confirmed just hip flexor strain (iliopsoas).

Okay, so as I bad as it felt initially (not to mention completely freaked out about the SF possibility), I thought it would take forever to heal. Surprisingly, I was back to running in 9-10-days, and ran a 50k pain free (with just a bit of groin soreness midway) 3.5 weeks after the initial injury.

I didn't do anything for this injury but rest it - I avoided anything that caused pain (even walking was painful, so I avoided it as best I could). I avoided stretching, and didn't take NSAIDs. When I was painfree walking, I started some VERY gentle stretching, which was a good way to test the injury. I would definitely suggest you take it very easy until you are mostly pain free - which could be much sooner than you think. Like I said, I had a bit of soreness when I started back, but not much. Take it easy, and don't start back running too soon.

Your symptoms sound similar to mine; my hip flexor isn't actually torn or badly strained, just way too tight. (Tighest my physical therapist had ever seen, she said).
Here's a test: lay on the floor, with your back nice and straight down against the floor. Grasp your thigh just above the knee (on the leg that ISN'T sore), pull it up (not hard, just move it) toward your chest and hold it there. Now, can you straighten your sore leg so that it lays down flat on the floor?
If not, your hip flexor is too tight.
I've yet to figure out what causes it, though.

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