That would be an infection. Most definitely.
I'm guessing it's the work of bacteria, maybe lactobacillus but I really don't know. Let it go and if it grows filmy bubbles then lacto is what it is. That will make it sour.
If it grows fluffy hairy balls all over, it's probably wild yeast, maybe brett. That will make it taste funky, but not sour.
Any way you slice it, it's not gonna be the beer you were hoping for. Me, I'd run out and get some roeselare and some canned tart cherries and sour it up and push it into a cabinet for a year.