Assuming that nothing about the setup of your guitar has changed, have you stretched the strings in? What I do with new strings is, change them one at a time - DON'T TAKE THEM ALL OFF AT ONCE! Replace each string in turn making sure you get a couple of wraps around the post, tune it up to pitch, then grab hold of the string above the 12th fret and pull it up away from the fingerboard to stretch it. It usually goes flat by a couple of semitones, so then I repeat the exercise (tune it up to pitch and stretch it) until it stops going flat. Be careful not to pull too hard and break the string!