Probably just a cold.
If your legs are equally sore then it's not likely this problem but strange moderate to severe backaches sometimes mean a lung blood clot.
I had that once, I swore it was just a really bad muscular back ache, it felt like when I got a backache from doing heavy squats. I never heard of such a thing, what a shock!?
They treat it with meds and it goes away but before the meds start it is a dangerous condition.
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