I think in the first or second chapters comments you can find but let me tell you it truly is something else. The two girls reasons and his reaction towards it are just... sigh...
Let me take a wild guess. The new girl knew the guy from before, typically childhood friend where they promised to marry or he helped her in some way (she mentioned him being like a hero in this chapter, that's probably a chekov's gun), she is afraid of all guys, specially drunk ones (could go with a tragic backstory of a drunk father or something like that) so she always surrounded herself with girls, but she likes the MC.
The former girlfriend actually likes the MC too, but she wasn't as innocent as she looked, the new girl found out and blackmailed her to stay away from the guy she likes. They agreed on that 1 month bet as well, if the guy remained "loyal" to the former girlfriend after a month, even after she dumped him like that, it means he would accept everything about her.