Yeah kind of figured something like that, palm muting the top two strings can put a lot of wear on em, specially when playing a metal type power chord song. Depending on how hard you're laying the palm down on the strings could be a big factor. Yes I would give the strings at least a couple of days to settle in before power chording them.
Could you tell me what type and guage you're using? Maybe you want to go for a heavier gauge. And remember you don't have to play harder to get the sound louder
Good clean pic execution at the right angle with the pic and the proper volume and distortion effect will give you the sound you want without having to hit the strings hard. And yes those strings can get expensive after a while especially when you play metal type music.
Oh and maybe swith to a thin pic. not medium or hard, the pic will bend easier allowing you to take some of the strain off the downstrokes.