Lots of good advice so far here.
I'm thinking you are getting hidden calories creeping into your diet. It's very easy to tell yourself that this or that didn't count because <fill-in-the-blank>. Every calorie counts.
As has been mentioned, get tight/strict with your calorie counting (at least for a few days) and you'll start to see the source(s) of the hidden calories. It's not until you know who/why/when/where/how regarding the gremlin that you can actually deal with the gremlin.