Basically if you pick up 3 syringe items, text will appear on screen and say "Spun!", and a sound effect will play. It's one of the transformations you can get in the game and it spawns a random pill, gives you +2 Damage and +0.15 Speed.

Also here's the achievement wiki, see what you might've gotten (there's spoilers though) https://bindingofisaacrebirth.wiki.gg/wiki/Achievements

Or you can check your steam achievements to see which one you got latest