No idea, but have you been pushing yourself too hard too fast ? You can't just throw your body into something new and not expect it to scream 'hey what the hell are you doing to me'. Personally I'd try and gradually get back into it, don't rush it but it might just be a case of easing back in and it'll stretch out once you're back in the swing of it.

Doesn't really answer your question but it's the best I'd got.

I wish I could help but I have no idea what it could be. When you returned to running did you go right back to the mileage you were doing ? You might have to cut back your mileage and then slowly work your way back up to it.

I've been running for about 3 years now after completing the C25K program. I had hip shenanigans on and off for a while and never really figured out what it was. These exercises (and getting OUT of Nikes and into Brooks shoes) changed my life --

1) Hip extension on all fours. On all fours, extend one leg back behind you, lifting it high so that foot goes well above butt height, but keep the leg fairly straight (not perfectly straight, but almost straight). Hold it there for a solid second, then bring back to past original position so that knee is ahead of the support knee, then extend the leg back behind you again. Do 8 repetitions each side.

2) Hip abduction on all fours. Picture a male dog lifting a leg to relieve itself. Lift your thigh out to the side so that the thigh is parallel with floor, leg bent at knee at 90 degrees. Your thigh should be perpendicular to your trunk. Hold for one second, then return to start. Do 8 reps each side.

3) Lie on back, legs straight. Pull one knee towards chest so that you feel a stretch in the opposite hip area, where the leg merges into the pelvis. Hold 30 seconds. Switch legs. Do two sets each leg.

4) Standing external hip rotation. Place hands on something for balance and stand straight. Lift one knee in front of yourself like a high march. Then bring knee out to the side, so that thigh is perpendicular to trunk. Leg is bent at 90 degrees the entire time. Bring back to original high march position. Do not set foot down; keep rotating knee outward, then back inward. Do 8 sets each side -- twice.

5) Standing hip rotation. Stand with feet shoulder width apart, hands on hips. Rotate hips five times in one direction, then in the other direction. The motion is similar to that of doing the hula hoop, except it is much slower, to get a good stretch all the way around.

6) Standing hip extension. Place hands on something for support. Stand straight. Lift one knee up and extend leg out behind you. It's okay to lean forward to allow this motion. This exercise is similar to "hip extension on all fours." Do 8 reps each leg.

It will take 8-10 minutes to complete these exercises. Do them once or twice a day (I did mine once a day). And I don't stretch before I run - I stretch AFTER.

first thing that comes to mind is stretching. laugh at me now but stretch when you wake up in the morning, throughout the day, and when you go to bed and you'll be amazed at how better overall you feel.

and you prob already know but stretch before and after you run VERY WELL