More running, same diet, more weight?!? help!

I run 4 times a week, normally about 5K. During the past week, I changed my route, and now do about 9K. I haven't changed my diet, but have put on 2lbs!!! Why has this happened?? I'm in the middle of my cycle, so can't even blame that either.
Can anyone help?

Muscle maybe

maybe you gained muscle weight, and that is heavier than fat.
measuring weight is not very helpful, use the mirror and check on your body fat percentage