M.P.
I suggest that her body is fighting off a bug. Let her rest and don't worry. If this continues for weeks, then take her to the doctor. For now, just accept that she's tired and know she will get better.
All of us feel better some days than others. I've learned for myself that when I'm tired and need more sleep that I need to get more sleep or I will most likely come down with a cold or the flu.