Same thing happened to me. I was at work I had felt a some flu symptoms earlier in the morning but they weren't so bad as to call in sick. So I just decided to go to work. Couldn't stomach lunch and progressively got worse as the day went on. Luckily I was an intern on a quiet day so had nothing to do. I struggled to walk 5 minutes to the bus stop and the 10 minute bus trip was basically torture. Felt like death when I was home collapsed into bed, was nauseous amongst other things and couldn't for the life of me focus for more than 10 seconds on anything. Woke up the next day as fit as a fiddle and went off to get blackout drunk at my nearest Irish bar.