In reply to robnights:
I'm in a similar spot - hip flexor on my R side has always been a bit dodge and was giving me a lot of discomfort at the end of last week.
My approach, given that I had had physio on it before and have done a lot of pilates, is to treat it with stretching, strengthening, and judicious use of a foam roller. I'm confident that it's not an acute tear, but there is a lot of scar tissue from previous groin injuries. Now 3X 1 hour sessions later it's already feeling a lot, lot better.
I'd agree about going to see a physio; they might also help with massage or stretching that will give some short term relief, and should set you on the right path.