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Not a great start to the New Year!
Top tip a physio gave me for helping with hamstrings. Fill a paper cup with water and put it in the freezer. Once it's frozen, tear a strip about an inch off the top of the cup. Holding the cardboard, use the icy end to massage along the length of the hamstring. You can put it back in the freezer and use it several times, tearing off more of the cup as you go. Works a treat with calf problems as well.
I'd bet stretching before you run contributed to your hammy problem. If you have to stretch before you run, make sure it's dynamic stretches, not static.
If you ice it, do some light stretching and some self-massage, it should clear up quickly.