Yeah. That's what happens. Just comes out of nowhere. Thankfully, it tends to be fairly short lived and just gets better as quickly as it comes on.
I once even went to work with flu once; Friday, my last day, came and I felt a bit coldy. Then in the afternoon, my new boss rings, "er.. I know you're supposed to start Monday, but we're a bit stuck. Can you start tomorrow night 21:00?".
Not wanting to leave a bad impression, the reply came, "No problem".
Saturday night I was feeling not too bad. At least that was how I felt when I went to work. But over the next 6 hours, out of nowhere, comes a raging fever (over 104), chills, coughing up thick blood-laced crap, and I feel like there are badgers trying to tear their way out of my chest. There's the added problem that I couldn't stand up either, because I saw stars and felt all dizzy. Somehow, I made it through the night, with only one minor injury from keeling over and hitting my head on something. 09:00 came, and I was out the door as fast as I could go (which wasn't very fast), stopping only to say, "I won't be back tonight".
36 hours later, and I was fine. Although the intervening period wasn't terribly pleasant. Lots of fluid, and alternating doses of tylenol/advil every 3 hours.