"iphone synced with another library" it isn't; please help.

Hi all

Tried to put a new CD onto my new iPhone 6 via iTunes.

Music moves over to Itunes no problem, but refuses to move to phone. Greyed out with a dashed circle to left of each track name.

Tried to move music manually; says "The iPhone “Andrew iPhone6” is synced with another iTunes library on “OFFICE-PC”. Do you want to erase this iPhone and sync with this iTunes library?"

My office PC went 3 years ago; I've checked, and I only have my Mac authorised on my account.

PLEASE HELP. I'm going mental trying to figure this out!

I love Apple but am struggling to see how iTunes is intuitive. Everything else Apple is so simple, iTunes is utterly annoying & time consuming.

PLEASE NOTE:

iMac running latest version of Yosemite....though interestingly it says "135122" where it should say Yosemite?? V10.10.2

Brand new iPhone 6 running latest IOS 8 version. (obviously never had it when I had my PC 3+ years ago)