1. Your strings stretched while they were under tension. You would be able to tell if this happened because they would have been out of tune.
2. Your neck was exposed to a change in humidity while you were gone and warped just slightly. This usually takes a day or two though since hard maple isn't a very plyable wood. Also, I don't think this is the case because the guitar is brand new and this normally happens to guitar necks that aren't lacquered. Fender puts some type of finish on all it's neck to protect it from this type of warping.
3. The guitar is brand spankin new and the truss rod shifted a few milimeters due to the string tension. This is rare, but sometimes when a truss rod is fitted it won't be totally locked down. Then after it's under a load for awhile it will finally shift into place. The guitar would sound out of tune for this as well.
Things to counteract this. You were in the right to adjust the action if the strings are buzzing... however... you were doing it the wrong way. You can adjust action from the truss rod, or from the bridge. You chose the bridge, which is okay and probably the proper place to start since your strings are buzzing even when they're open. You started by turning the screws at the end of the strings (philips screws) these screws are for adjusting intonation... not action. The scews to adjust string action are the hex screws on the bridge saddles. These are the screws you should have turned. As far as adjusting the action from the truss rod... you can use the procedures above. If you don't feel comfortable with any of these procedures, or don't know what we're talking about... just take it to a tech man... You might have already messed up the intonation. So I suggest taking it to someone who can fix you up.