I know my mantra is cheap science, and by no means I think a $3.50 bottle of water from the airport is "cheap", but when you spewed your guts out 11 hrs prior to hopping on a flight, sometimes you have to bite the bullet.
Anyways, I was drinking my water at cruising altitude, closed it, and then slept for the entire descent.
This is what happened to my water:
It was squished, but I didn't squish it! Why did it squish? Well, the pressure up at 30,000 ft (approx cruising altitude) is much lower (~4.3 psi) than it is at sea level (14.7 psi). We're basically sea level where we live in CA. As we descended, the outside pressure increased, putting pressure on the outside of the bottle. Since the air inside the bottle can be compressed by forces caused by the change in pressure, the increased pressure basically crushed the bottle. *Note, that the airplane cabin is pressurized, so we're not feeling 4.3 psi, though I'm not sure what they pressurize it to or how they pressurize it.
I opened the bottle in the car and it immediately equilibrized to the surrounding pressure, popping the bottle back in shape.
If you reverse the experiment and do it on the way up in the air, you might find your bottle pops open by itself as you get higher. This is because the air at sea level has more pressure and is trapped in the bottle. The air up high is lower in pressure. The forces are higher inside the bottle in this situation. So if your shampoo explodes in flight, it most likely did it during the ascent (going up).
This water bottle experiment has amused our little flier on multiple occasions.