Is it possible?

Today was a swim time trial. 500 yards, so not very long. According to my watch I did it in 7:31. Is this possible? I have been averaging about 1:40 - 1:44 per 100 on my time trials. I would expect that I could do a little better on a shorter distance, but 10 seconds per 100? I did push real hard. I intententially started hard - as if I was swimming 200 yards only and then tried to hang on for the remaining 300 yards. I was in extreme oxygen debt by the time I finished almost to the point that I was going to black out (don't worry, pool's only four feet deep). So I ask again, is it possible that I took 10 seconds off per 100? I have another 500 yard time trial next week so I am going to find out. I wonder if I can find someone to count the laps for me....................