I'm new here so bear with me

I used to run 4- 6 miles a day through out my twenties (I'm now 33). I was in a car accident a couple of years ago so I had to lay off the running and took up the use of a recumbant bike and began yoga and pilates. In the last year, I began to work out more intensly doing step aerobics and weight training and occasional short runs. I really miss running and have planned on doing a 5K in April so I have slowly started running consistently again (just 3 days a week for 40 minutes on a treadmill until the ice melts and I can go outside again) and now I am having terrible pain in my hip flexors to the point that my hip feels like it wants to "give out" on me. I have never had this problem in the past, I'm always very careful to warm up, cool down and thoroughly stretch. I still see a chiropractor and masage therapist but neither one can detect anything wrong.

Any ideas as to what might be different this time around and/or what I can do to ease the pain?

Thanks!