A very similar thing happened to me. Two pound loss the first week, and then a three pound gain the next, with no change in the amount I was eating and exercising to account for it.
I was thoroughly demoralised and sure I must be doing everything wrong, but in the absence of a Plan B I stuck with what I was doing.
Sure enough, the next week the three pounds was back off again, and other than another blip one week when I got a bit careless about logging food while I was away on holiday, it was a slow but steady loss of one or two pounds a week from there on. To this day I don't know what caused me to put on weight the second week.
So don't be too downhearted. Stick with it (which you sound like you've made up your mind to do anyway) and I'm sure you'll see the weight starting to come off.
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