Accidentally Warping Trays with Whitening Gel?

[phasesofbeauty]

I recently got Vivera by Invisalign retainers after my Invisalign Express treatment. I have heard that you can use these for whitening. But, will this cause the retainer to stretch out or distort? I have done online research and heard that plastic retainers eventually distort or warp (which is why you get multiple Vivera retainers). I whitened my teeth about twice using the retainer and it felt tight with the gel on and not as tight after I had taken the gel off and brushed the retainer. Did I distort the retainer or am I being paranoid? I have all four of my retainers at home....should I switch to another one in the set of 4?