Backing track in the vocal track

Sorry for a stupid question, but I have a problem with backing tracks coming uninvitedly to a vocal track. I’ve been doing this a long time without seeing this problem and looking back to my old recordings, vocal tracks includes just my voice - but now when I try to sing along the backing it all come blended to the vocal track.

This is clearly some setting mistake - but I can’t find it.

(Cubase Artist 7.5)

Thanks in advance,

Too loud headphones?
Open back headphones?
Wrong routing somewhere in the software or in the audio interface or in the driver settings?

We can just guess here…

Disable “loopback” type functionality in your mysteriously unmentioned audio interface?

Just a guess, given the few details provided …