Having the strings off a guitar for a few days shouldn’t damage the instrument at all. First, did you attach the strings to the tuning posts correctly? If not that will cause slippage. Secondly, if you went to a heavier gauge set of strings and you have a tremolo bridge that might be the issue. Heavier strings have more tension and will pull harder on the springs of the tremolo. Search “change strings Stratocaster” and “adjust tremolo Stratocaster” for some good info and tips.

I had originally deleted my answer but decided to edit it and post something because there seems to be a lot of focus on string slippage but first, steel strings don’t take long to stretch if they are installed correctly and second, stretching strings won’t cause a bridge to rise.

The fact that the bridge is rising tells me the real issue is your new strings are heavier and/or have more tension and are pulling harder on the bridge and tremolo system. You could try a lighter gauge set but if you want to use the ones you bought then you need to make some adjustments.

I suggested searching “adjust tremolo Stratocaster” in my comment. There’s a good chance you can solve your problem by either putting more tension on the springs with the trem claw, the metal piece the springs are anchored to that is held in place with 2 wood screws, or adding a spring if your guitar only has 2 installed. Watch a few videos and follow the advice of the various skilled luthiers who post useful information for us do-it-yourselfers.