Wow whenever Yamada has a doubt about Kase-san I always start doubting her too. But Kase proves me wrong every single time. Also kyaaa that kiss xD
In this case I dont think its fair of us to 'doubt' Kase-san realy- i mean while i can understand why Yamada would be jelous / upset if it WAS true, its not a crime or in any way bad to have had a previous girlfriend.
Im also though of the opinion others have stated that she probably hasnt had one- and its just rumours. I think it would be quite well known and said WHO her girlfriend was if she'd had one.
But even if she HAS, I wouldnt like her less. Yamada should just ask her about it if it bothers her that much :)
Either way I love this manga!