There is an entire thread from the January 21 Day that you will find very helpful. Here is an excerpt from something I shared there:
This happened to me as well. Actually, I gained 2 lbs in the first month. And this was after decades of vegetarian diet! So the changes for me were dairy, free fats, higher fat foods.
They came on pretty quick, stuck around for that month, and then in the month after I dropped them, then the next month 2 more...
Our bodies need adjust time. We always dread the idea of weight coming on fast but want it to go OFF fast, aren't we something?
If your changes include a step up in dietary fiber (usually does, did for me) then you have more fibrous content in the digestive track which - holds water. Translates to poundage, but not bodyfat poundage. So you may be losing fat yet holding a bit of water in the system.
Carbohydrate binds more water in our system than protein, so if your macronutrient content has shifted, this can explain water weight as well. I look at it as getting hydrated, especially for those coming off a dietary plan that is too steep in protein, which purges water from the system. You may be re-hydrating.
I hope you find this encouraging! Give it time, take care and have compassion for your body, and keep getting support for positive changes!