I put on 7kg in a month?

I've recently been away on holiday for a month, prior to this I was going to start a diet anyway as I have put on about 10kg this year. Well I knew I had gained more on holiday but I've put on another 7kg, how have I managed to put on almost the same amount as in 7 months, and what's the best way to start losing it, I'm feeling very fat now.
I put on 7kg in a month?
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