Why did I gain weight?

So I went to the doctor yesterday for the first time in over a year, when I got on the scale I nearly died. I gained 15 pounds. warranted I do have my period but that utterly ridiculous. I meticulously watch my calories, I exercise 6x a week. I do circuit training cardio and pilates, plus I'm an organic vegan. How is this humanly possible. People have been telling me I look thinner lately so I was really confused. How did I gain weight instead of losing? I'm 5'8 1/2 and currently 145 ewww