Weird fibrous mass formed from by beer?

I added lemonade to my beer and it foamed up into a strange fibrous mass, kind of like cotton wool. I pinched it out in one piece (put it in here for safekeeping, the droplets are from yoghurt not the foam). I can’t find anything online about this happening. I tasted the beer (now free of strange cotton foam) and it tastes fine, but goes from initial taste to aftertaste within a second where it took a few seconds before. Beer is McEwan’s Export, lemonade is BARR. Anyone know why this happened?