New strings take a while to break in on a Strat. You'll have to retune occasionally (after bending etc). "G" string always goes out first from bending. Strings stretch - basic mechanics. Hot/cold temp changes will also raise havoc on you're tuning.
Little "trick" for new strings - overtune a half turn or so, then bring the string back down (sharp to flat) into tune.
Lastly, as others have already posted, stay away from the trem bar on a strat - you're guaranteed to knock out the tuning....That's why Floyd Rose has a patent on the double locking trem...
-Bob