Why are my teeth turning yellow quickly?

I have noticed my teeth have become more yellow over the last year but only mildly. However, over the last couple of months, and especially the last few weeks, my teeth seem noticeably more yellow - especially the canines. Is it more likely that these are extrinsic stains that have been building up over two years or is my tooth enamel just wearing down suddenly and quickly? I have a dentist appointment next week but this has me racked with anxiety.