Why have I gained 6 pounds in one day?

Here is the low down.... Saturday I pushed mowed my acre in the hot sun. Ugh! That night I weighed like I always do and I was down a few pounds (Yay for sweating). Sunday I came off my birth control pill so that Martha Monthly (period) could do her thing (usually arrives on Wednesday). I woke up and weighed myself, 130lbs. This morning (monday), I was 136 pounds!?! WHY??? I've been told water retention but I don't agree. All I do is drink water all day and I drank extra water due to all heat and sun burn I got from mowing. I don't eat salt to be retaining any water either. I eat fresh fruit and veggies, I grill or bake any meat that I eat. I don't get why I have all of a sudden gained this 6 lbs. Can someone help???