Your body has been put through a lot recently and will need some time to regain that equilibrium. Were you happy with your size before the op and is it just the stone that you want to lose?
Careful with counting calories, although this is a necessary part of losing weight, it can be disheartening and an unhealthy way to view food. My advice would be to eat 3 meals a day and focus on getting as much quality nutrition into those meals as possible. Not only will eating 'clean' help you get back to your former weight but it will help your body recover from the op and all the meds that go with it and give you more strength and energy.
Make your meals vegetable focused with protein and starches as more of a side dish and you'll naturally eat a lower calorie diet whilst getting a lot of nutrition from your calories. Avoid processed foods, alcohol and sugar as much as possible.
What kind of exercise were you doing pre op? Exercise doesn't have to be intense - it's just movement. If you can walk that's great, if you can move your arms then you can do some upper body strengthening...so may options and I'm happy to help if you want more specific advice.