Eek, that sounds alarming although I've had that happened to me as well. Could be down to a few reasons:

1. Anxiety - you've a stressful time ahead
2. Blood sugar has taken a dive (I take you're not pre-menstrual though ha ha!)
3. It's the height of the hay fever season, I feel like I've been punched in the face today

I'd be inclined to stop when your body tells you to, if you try to force yourself through it, you'll cause yourself all manner of problems.