Quote Originally Posted by ic-racer View Post
In a closed container, there is an equilibrium between the pressure of the gas in the bottle and the vapor pressure of the liquid. The vapor pressure of the liquid can change considerably with changes in ambient temp. Thus causing the plastic bottle to 'cave in' when it is cold and 'bulge' when it is hot.
I think this is the most likely answer. If you, like I, mix your chemicals at the maximum recommended temperature, say 80F, and then seal the bottle and let it cool, it will contract.
Next time, let the solution cool with the cap loose and then tighten it, and I'll bet it won't happen.