Got my braces off today! But im worried they'll move overnight!?

So i got my braces taken off today! After 7 month, which is quite short, they are finally perfect! I do plan on maintaining the perfect image, once i get my retainer that is, however im worried! They took my impressions today and i made an appointment for tuesday to fit my retainer, the one thing that worried me, is what will happen over the days until my retainer is fitted? Will they start moving? If so will the retainer move them back into the current position or not? Someone help, im stressing over this because i really like them!