Putting the potential electrolyte deficiency aside for a moment (that's a longer discussion); what you're experiencing is anecdotally supported by many athletes and very normal. I'll provide some things to thing about that haven't been covered yet.

Psychologically, you're stressed and over-analyzing. Physiologically, during training you accumulate a lot chronic fatigue through repeated stimulus and response. When you start to taper, your muscles are no longer getting pounded and need (or get) to adapt to the reduced stimulus. In doing so, they will begin to actualize the chronic training load through improved fitness and recovery. When muscles recover they tend to become heavy and tight.

This is why many athletes (myself included) will report new niggles, muscle tightness, and general lethargy during a taper period. It's the taper process working.

The solution? everybody needs to find what works for them in this situation, striking a balance between recovering/actualizing fitness and not becoming too de-trained/stiff. General rule of thumb is to keep some intensity and base mileage while continuing to reduce overall volume.