Left Hip pain when cycling up hills

Hi. I wonder if someone could give me some advice please.
I have been using a road bike for 3-4 years and have been gradually pushing myself more. On average I do, weather and season permitting, about 100 miles per week. During this year I undertook three sportive events, 67 miles (Jul), 108 miles (Jul) and 89 miles (oct). After the final event I developed a pain on the outside of my left hip (juse below my pelvis), which feels like it is on the outside of the bone, which gets worse when riding up hills and putting in more effort.
I have looked at various cycling forums and am now regulary doing core strengthening exercises and a couple particulary to strengthen the piriformis muscle, however these are having little effect so far. I am not using my road bike at the moment, but using an exercise bike (lightly) and a cross trainer, and these do not agrivate the issue so much, when they do I turn down the resistance.