Weight loss venting

I weighed myself this morning and came in at 181 lbs., which is surprising (and infuriating) considering

a) I was 178 lbs. and change on SATURDAY!
b) I haven't exactly been gorging myself on bad food.
c) I ran 5K on Saturday, then walked 2.5 miles back from the car-repair shop, and then walked 4 miles on Sunday morning.

Yes, I know that a person's weight can vary quite a bit depending on things like hydration levels, but a 2.5+ lbs. variation in 48 hours isn't exactly what I wanted to see.