From what you have said, wrong stepping it sounds like your sacro iliac joint. sometimes a sudden jolt when running can be enough to cause an "upslip" in the ilium or pelvis. This can then cause presure or swelling on the sciatic nerve reulting in the typical horrible pain down the leg. If it is really only down the one leg then it is more likely to be either sacro iliac dysfunction. The causes of this can be anything from tight piriformis,hamstrings hip flexors or adductors. Or it can be in very many cases overpronation of the feet, this is where the arch rolls in, the leg then internally rotates and cause the si joit at the rear to be under pressure.
If the pain is bilateral or down both legs then it can be L5 S1 which is where the nerve originates from, this can be more serious, it will hurt just the same but the outcome can be more complicated. Areally good simple test is to,
Stand upright then lift your thigh to waist height allowing your knee to bend
Then take your leg behind you into extension, do not bend foward while doing this.
If the pain is increase significantly on either of these exercises, then it is more likely to be SI joint.
If bending from the waist causes pain and the cause is sudden then it is possibly disc at L5 S1
If leaning back causes pain it could be facet pain, the little joints on the side of the back.
Self treatment,
Anti inflamatories, stretching as the other guys advised and look at your foot strike in bare feet infront of a mirror, if rolling in,get orthotics(insoles).
If the symptoms don't settle down within a few days, its worth getting treatment. If treatment does'nt resolve it and the pain is constantly radiating down the leg it is worth getting an MRI, at present private £200.00 ish.
Hope it helps southey
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