26 pounds in one week? No way. That's impossible
I think it's entirely possible. I lost 17 pounds (135lbs to 118lbs) in one week, when I was in my late teens. I had just come back from several weeks visiting relatives, where all we really had to do was sit around (I think I knitted an entire sweater during one of those weeks of the visit) and eat and eat all day long. And it wasn't healthy food, either... it was cinnamon rolls, cookies, and a bunch of fried crap.
When I got home, I rode my bicycle a lot, and ate considerably less. Within a week, I'd lost a bunch... and I didn't feel like I'd starved or overworked myself. Although - I was an invincible teenager still.