Along with what LATS said... if you changed the strings you may have not fully installed them correctly. People often skip over the last step of changing strings.... The stretching step. After you get the strings changed and you tune them to pitch take a string (one at a time) and pull it around in all different directions. It may feel like the string is going to break but it won't unless you really rear-back and tug on it. Metal will flex/stretch before it breaks... that's why you do this. This relaxes the molecular bond in the metal by releasing the stored energy within it. Strings break when they reach a point when all the elasticity has been used and the molecules along the weakest point of the string have lost their bond... this is known in engineering terms as the modulus of elasticity. When a metal approaches it's modulus of elasticity it experiences a point called climactic elasticity. This is where the metal has released all the stored energy within it's bonds and the metal is at it's most stable point. By stretching the string you help the metal approach this point without having to play for hours and hours on end with your guitar coming out of tune every 5 minutes. Sorry for the engineering explanation... but I'm at work... so I'm kind of in the mode. But in short.... stretch your strings after you change them, it will help them stay in tune.