weight gain problems😩😩😩😭

I'm 28 and all of a sudden with the same exercise and food intake I've gained A LOT of weight. (20lbs) I've been slowly gaining since about December. I've been working out more so I stood on the scale today - it's 10lbs more than it was 3 weeks ago!! 
I know switching to a desk job has changed my life .... but I'm wondering if my metabolism is changing as well. 
What age did you notice your metabolism changing? What changes did you make?