Hi Guitar players.
I'm a novice. I have a yamaha fs100 guitar.
I store it in a case. When I take it out to play, it always seems to be out of tune. I retune it, but after a few minutes of playing it goes out of tune again. I've changed the strings and it does the same thing. I use a Guitar Tuna app. When I look at the reading it never holds a straight line for long and can say it's too high after I first play the string and then says too low as the sound gets quieter.
I think it might be something to do with the temperature change from being in and out of the case.
Is this possible? Or is there something wrong with the guitar.