She is extremely insecure and it starts from way before. She was initially in bro zone with all the guys so she heard their conversations about Shamita. She herself knows all the guys liked Shamita , she herself told Shamita once when they were in good terms that all the guys like her and her upset she has a bf. Moreover she asked KK once if Shamita reminds him off his ex and also admitted that she is definitely his type. So now that she is with him all of this is resurfacing . She knows she is not his type and probably was the only option here.
However, its okay to be insecure about someone but to be so jealous, vile and make that person's life hell with stupid allegations is on another level.