Weight gain so quickly

Hi I reached my goal weight before my holiday by eating under 1000 Cal's per day . However I have just got back and have put on 9lbs in 3 weeks!!! I ate and drank loads but also did a lot of swimming and walking and it was 38-40degrees.
Is my body always destined to be on a diet? Will I ever be able to eat normally.
I'd say the problems it that you ate really few calories. In my opinion less that 1000 cals a day is not enough and in order to lose weight more consistently you should rather try to stay within a healthy calorie deficit. It's important that you really change your nutrition and not only do a radical diet that you're not able to maintain.