'Flu-like illnesses can do that. A couple of times I've had something like the 'flu but with no cold symptoms - just the weakness, loss of appetite, raised glands and alternate shivers/sweats.
If you go on the bike and have a HRM, wear it and make sure it doesn't go to stupid bpm on hills. These infections can muck about with your MHR and allow you to push yourself beyond your normal physical limits, which can cause you to black out. It happened to me in January - two days later (after being checked out) I was back on the bike and my heart rate was touching 186 up a pretty modest incline.