I have wondered the same, personally I don’t think it’s anything to do with your exercise bike unless maybe you have over done it. I seem to get days like you describe, just don’t feel well but can’t exactly pin point the cause, you think ah great no fatigue I’m going to have a good day today and then you can’t do anything cause you just feel unwell.
Im in the menopause at the moment and am 48 and have to put it down to that can last for just a day or a few,I wonder if this is the norm for us with Ms.
Im not on any DMD’s either
I do know that the menopause causes aches and pains and headaches due to the lack of estrogen.