I visited a physiotherapist today, and this is what he diagnosed. Luckily it's treatable, lots of ice some gentle hamstring exercises and hopefully I'll be able to gradually start running in 3 or four days, but only 10 mins to begin. We also talked about cold water, and he says 1 min cold, then 1 min as hot as you can repeat 7 times.
Can't wait to get better so that I can try my new trainers (before the 30 day exchange time expires). I know I'll have to take it easy for a while, so slowly slowy does it.