Gaining weight

Since June first I've lost roughly 20 pounds, however, recently I hit a plateau and in the last three days I've gained about a pound and a half a day. I'm going to the gym every day, still in a 1,000 calorie defecit. The only change is that I've ate some red meat and pork products since I've had family in town who don't follow my diet. How is this weight gain even possible?

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