I'm sorry you are experiencing this gain.
You've done fabulously! I never heard of this happening. If your doctor doesn't investigate further, I would definitely get a second opinion from an endocrinologist. There are a lot of reasons for retaining fluid, which it sounds like you are experiencing.
Something is going on besides eating back some weight, so I would avoid P1 and try your maintenance plan that worked for that first 4 months. If you gained back a bit of weight from holiday treats, it might be better to address that after you know what else is going on.
Fwiw....when I phased off, I had some weight gain (from eating too much). It scared me back to P1, and then I began a crazy restrict-binge cycle. I had to stop doing P1 for good to stop bingeing.
I hope you get this figured out soon. You did an
incredible job of getting to a healthy weight. Hopefully this is just a little bump in the road to a comfortable maintenance for you.