Lots and lots of things can cause tuning changes or instability.
Like SinSir said, new strings can be the reason behind the shifts in tuning stability. If the strings haven't experienced enough stretching, they will slip during and after playing, causing tuning changes.
Other than new or old strings, tuning changes can be attributed to climate or temperature changes. I have experienced this myself. For instance, playing the guitar in one room and storing it in another with a cooler or warmer resting temperature will cause the tuning to slip.
Further culprits...a proper setup is needed for solid tuning stability. Making sure the bridge is level, the nut is properly cut, using the correct gauge strings, not having too many (or too few) wraps around the tuning post, and having a neck relief set within spec. All of these can affect tuning stability, both while playing and while your guitar is resting.
As we ask more questions and get your feedback, we may be able to narrow down where your tuning issue is falling and help you solve your issue.
