Purple Plums on an Apple Tree?

[Balkanicity]

Now, you're going to think I'm a few cents short of a Euro....but anyway...I planted an apple tree about 3 years ago and much to my delight I had two of them last year, nice big red juicy ones (can't remember the variety, but you know...the all red ones, yum) Anyway, this year I was delighted to see more fruit growing on the tree, small though it is....only...they are purple plums.....huh ?