Let's say there is a female adult. She used to have a slender body, but now she is a bit overweight. You are wondering what might be the reason for this, and then you have found out that her Facebook photos are full of different fatty food as she has become a careless foodie. And you say:
"So that's the reason why she gained some weight!"
Is this grammatical? In terms of the verb tense I have used? Should it be a past tense or simple present tense?