iTunes on pc keeps deleting my music

Really hope someone can help. I use iTunes on my pc and have done for years to burn CDs to iTunes and then transfer them to my iPhone 6, recently that phone ran out of space so I bought a new iPhone 6S 128gb, I do the setup using my old iPhone 6 so that it copies all my settings, music etc to my new iPhone 6S. Today I plugged my iPhone 6S into my pc for the first time to put some more songs onto it but iTunes deleted all the music off my new iPhone. This never happens with my old iPhone 6, but it keeps happening with the 6S, I’ve reset the phone 3 times and done the setup again using my 6 to copy everything to my 6S, on iTunes I have the same boxes ticked and settings applied that have always been on there but I cannot stop iTunes deleting all the music, I chose not to sync and then that way when I plugged my 6S in the music was still on the phone but as soon as I transferred a new cd to the 6S it deleted everything again but did transfer the new cd. Does anyone know why this happens and if there’s a way around it? I have over 1000 songs and I really don’t want to start from scratch burning CDs and transferring all over again. Thanks in advance