Are you sure you were all the way in stealth? You now have to go a full 1.5 seconds consectuively taking no damage for stealth to fully activate, and it turns off the moment you deal any attack.
Are you sure there was no pink ward around? One could be in a nearby bush which would cause you to be revealed even if you can't see the pink ward.
Were there any champions with abilities that grant true sight on their team? Here's a list:
http://leagueoflegends.wikia.com/wiki/True_Sight#True_Sight
Something like a Lulu casting Pix on you or a TF ulting can grant true sight of you even through stealth.
Were they auto attacking you or using targeted abilities or were they only hitting you with skillshots? It's possible they were merely guessing your position correctly or aiming based on other skillshots that show 'hit' animations when hitting you in stealth. They can also have your position revealed by a red trinket showing your outline (but still leaving you untargetable) allowing them to aim their skillshots at your position.