i have a similar problem and have been assessed as having sacro-iliac joint problems. basically my trans abs muscles are weak and so are my gluts (bum) therefore my hips and sacrum are free to move around. the body can with stand this for so long and then suddenly when you up mileage or speed it 'goes'. i haven't been able to run for 2 weeks and its doing my head in. my sister is a physio (i'm a podiatrist) and has given me strengthening exercises to do, pilates and i am going to try a sacro-iliac joint belt. maybe this could help you. i originally had the pain a few months ago, changed my shoes and was fine for about a month. then it came back worse than ever. don't want to scare you but definitely go and get it sorted sooner rather than later, so you can run off into the sunset........ kathryn :)