This isn't an unknown thing really, in fact it's pretty common. Usually happens in the morning in the space between when I take the medications and when I eat something, so I've been thinking that it's related to not having eaten anything (not the same as low blood sugar), or more that the cells are not getting the energy they need.
It has become more pronounced since I've been riding the bike again. Today is the worst it's been that I can remember.
It could have something to do with all the sinus/hay fever stuff.