When I picked up my guitar today I noticed my 1st string was a bizarre 2 semitones flat. I would tune it but the instant I bend it, stretch it, or play for a minute the string goes back down to the exact same note and won't hold tuning any higher. Granted the strings are 6+ weeks old and I play 6-7 days a week but this only occurs on one of my guitars. I'm using GHS 9's in drop C (easy on the fingers).
Why do the higher strings lose the ability to hold certain tunings? Is there a reason it always detunes to the same note? Is there a way to prevent this so I don't have to change my strings so suddenly?(I only want to change them when I record something high quality).