If it's just tight and achy, you may want to just try stretching it more, doing some leg lift type exercises, and getting one of those foam rollers that you lay on and rolling on your thighs and IT band.
If it's causing you significant pain, I'd recommend seeing an orthopedist to rule out anything serious. (Edit: a number of martial artists I know, myself included, have had to do PT or even orthopedic surgery because of torn ligaments, damaged cartilege, etc.)
I'm not sure what hip flexor pain is, to be honest.
I do have some hip pain these days and I notice it particularly when I am doing weapons training - kobudo stances are wider and deeper in my style than empty-handed stances. I believe in my case it is arthritis, but I do plan to mention it to my doctor next month during my annual physical.