What would make the same workout way Harder sometimes?

For the last few months I have been walking on the treadmill for at least 30 min 4 days a week. The last few weeks I bumped it up to 40min and now I'm going up to an hour. The last 3 days (including today) I have walked for one hour a day all at the same pace and incline but today it was immediately more difficult than the previous 2. I was so quickly winded and sweating, I was ready to quit after just 15 minutes. What would make it so much harder to do the same workout today than it was the last few days?