From what I have read, that is too few calories for someone three months post op. You won't have put on fat weight, it is more likely fluid retention plus possibly bulk from the extra food. It is only temporary, a bit like when people go through the post op diet stages, as they progress they typically have a stall or gains, but once their bodies adapt weight loss kicks in again.
While WLS does typically result in a lower calorie intake, you don't want to go so low that it comprises your health: body using muscle for nutrition - you want to maintain muscle as much as possible, hair loss, lack of energy etc.
Bear in mind, now that you are already in the healthy BMI range, that weight loss will be lower per week than it was immediately following your surgery.
Please do listen to your nutritionist.