Hip injuries can be tough. I've suffered through several issues with my IT Band and hip tightness. In all honesty I had the best luck with PT that involved a combination of strenghtening my glute (butt) muscles and using a foam roller and rolling out my hip and IT band. The switch from a heel strick to a forefoot strike is most likely putting stress on your quads that you're not used to. If your quads are tight, then they're going to pull on your hip flexors - causing you hip discomfort. Another thought is that a lack of glute strength is causing your quads to over compensate. It sounds like this could be a strength/flexibility issue.
Hope this helps!
You seem to have the exact same IT band issue that I am working my way through. I'm 43, not overweight and have been running about a year.
I had an intense pain in my left hip that came on slow but got worse every time I ran until it was unbearable. The pain was tolerable when I ran but would become intense a few hours after. Fine when sitting, but pain when walking and especially going up stairs.
I've had success so far (ran a half marathon and 5k with no pain after) by doing the following:
1) Stopped running long enough for it to heal and the pain to go totally away.
2) Durring that time I streatched every day focusing on my glutes, hamstings and calves. I also used foam roller everyday to roll-out the hip and IT band. This was extremely painful at first.
3) Now that I'm running again I am continuing to streach everyday and also before and after each run. I use the roller after each run, it still hurts but is much less painful than before.
4) For the longer runs, I've also added a stop to streach my hamstrings half-way through.
I dealt with 6 months of hip pain that I thought would never go away- first I went to my chiropracter for active release therapy thinking it was my hip flexor. the therapy would relieve the pain a bit when it returned with every run. Then I went to a orthopedist and he told me not to run for 6-8 weeks bc it was a "muscle strain". I think they use that diagnosis every time they don't know what it is. Needless to say, I did not heed his advice. The good thing he did was send me to physical therapy. The PT first realized the pain in my hip was coming from my lower back NOT the actual hip joint. She found a motion that relieved some of the pain almost immediately but each time I ran the pain returned. The motion was similar to the updog motion in yoga (http://www.yogajournal.com/poses/474). After we discovered this exercise that relieved the pain I started to go to yoga twice a week and within 3 weeks the pain was GONE. Never returned again. In conjunction with yoga I got new sneakers (after having gait analysis since the hip pain made my stride change) and a pair of insoles (just super feet for extra support). I continue to do yoga at least twice a week. I love it for many reasons including injury prevention, improved strength in my hips, back, arms, core and just the overall relaxation of it.