I ran 28.5 miles & gained 3lbs. WTF!

Yeah, I finished my 1st 6 hour ultra event and ran 28.5 miles. When I finished I was on cloud 9! That feeling soon ended when I  got home, showered, and weighed myself. I had gained 3 pounds! WTF!!!  How is it that I can run so far yet gained 3 pounds? This is absolutely...