Doesn't Yash's brother's drink? Or is that just me imagining it? If yes, then Yash would have drunk before right? And what about Aarti? I may be a bit
biased and say that to me she doesn't look like the one who would drink bhaang or other intoxicants..
Theoretically, those who have drunk before are more...I guess used to their effects than those who don't drink or drink less...
Yash may remember because he is used to the effects and realised what happened the morning after...Aarti, who may have had a hangover, (I'm not exactly sure as I don't drink/never have) and therefore would have bad memory of what happened...also, it may mean that Yash had tried to hide everything that happened...such as by tidying up (as he does have some form of OCD) etc etc...
Though, the one thing that doesn't make sense is how Aarti didn't "feel" that something happened the morning after...(Again I've never been through this so I'm not sure..)...but maybe she'd been concerned with the headache or hangover whatever from the bhaang that she didn't take notice and then gradually forgot?