Sometimes it can just be an energy slump/one off/ocassional thing you will comes across that you can't really explain.
Make sure you are well hydrated before your run and a while before have a snack on something high energy like nuts perhaps. The heat at the minute can take a while to get used to running in so that may have had an affect also.
You perhaps aren't giving your legs a chance to recover, they do say a rest day between running days is best. That doesn't mean you can't do some other form of exercise though. Even doing a gentle / moderate brisk walk would be better than running two days in a row.
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