Weight gain on the pill

I've been on Cerazette (the mini pill/progestogen-only pill) for about 7 months now. Since taking the pill, I've found that I am gradually gaining weight and struggling to keep it off. A couple of years ago, I was obese and managed to shift 2.5 stone to get myself to (and maintain) a healthy weight. I've now shifted back into the "overweight" category and I'm struggling to take control and lose weight, despite using the previous strategies (calorie tracking via MyFitnessPal and walking 20-30 mins per day). I spoke to my doctor and she said that all evidence of the pill leading to weight gain is "purely anecdotal", but my weight has only started creeping since the pill. I know that exercise is a game changer, but time is a problem for me: I work full time (in a very static desk job!) and am studying for a masters degree on evenings/weekends, so I am forced to spend a lot of time sitting! Does anyone have any tips?