Actually, I've had the same problem while sitting at my computer. My DH had it in bed (after having been sitting at the computer). Turns out we have very tiny ants that are extremely hard to see, they're almost clear. But they have a big bite!
Next time you sit down to sew, have some scotch tape handy. If you feel a bite, stick it quick with tape and see if you don't catch one of the little buggers.