My back suddenly started hurting and...?!

On thursday in the afternoon in P!.E!. we were doing this running activity where u run,skip,sprint or whatever the teacher told us to do and so at first i was fine and then a half an hour later my lower back on the right side started hurting soo badly!. then it went away came back in the morning went away came back in P!.E!. even worse then the day before and it's been hurting ever since!.
i wear the right type of shoes for running too!.
do u know what this could be caused by!?
thanks in advance!Www@Answer-Health@Com

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