If you follow Richard Purkis's log, cause of his ongoing HF injury was cross-training...
I definitely haven't done enough of it to cause an injury!
I'm guessing Richard is bored of reading about Hip flexors.
My advise would be to get it diagnosed straight away. If it is a strained Hip flexor give it a rest. My problem is that it felt fine before a run (because it is not being used during everyday activities) and only when you're raising your leg whilst running is it engaged and becomes aggravated.
I worry that a cross trainer might be "using" the hip flexor so that me be why you felt it the next day. When i fist did mine I just assumed it was only a small strain and i thought a few days of cross training would be fine but it took me ages to realise what i had done/ what was aggravating it and not letting it heal. I would hate to see you have to sit out for two months like I have been with the amount of races that you do.
Thanks for the advice; I'll see how it goes but will bear that in mind :)