iphone keeps deleting music?!

hi, can anyone please help me as this is getting very annoying! 😠

everytime i plug my iphone into my mac at work to charge phone and play music from it - through itunes - it starts to automatically delete the songs off my iphone?!

I've gone through all the settings on the phone and in iTunes to stop any sync or backup between the two but it keeps happening everytime i plug and play??

its becoming very annoying as i'm finding myself having to resync all my music back to the phone most nights in order to have something to listen too!!!!!

I've never had this problem before with previous iOS' and having gone through every setting i can think of to stop this from happening, i'm stumped!

any help would be much appreciated

thanks