I'm one of those people who doesn't believe the "starvation mode".
To OP: 10 lb in 3 days is absolutely water weight.
1. On or nearing your period? We all know that TOM can cause bloaty-ness, aka weight gain.
2. Exercising can cause your muscles to retain fluid. This is "exercise" weight, or what many people say "muscle weighs more than fat!" (which no, it doesn't...
a pound of muscle and a pound of fat is still ________. (answer: a pound!)
3. Sodium. aka The Devil! - sodium is the main reason why we gain/lose weight seemingly overnight.
While none of us like to see an increase on the scale, keeping that whole water-weight thing in mind can help us not get too crazy over it.