eating less, gaining weight?

Nonsense.

Okay, no confirmation, hence, no explanation.

<<Disclaimer: these are just musings on a topic that may interest me, but that I have absolutely no special knowledge about>>

When I read the question, the term "water weight" floated up into consciousness from some deep recess of my mind.

It certainly seems possible that your body could gain a bit of weight temporarily by taking on water as some reaction to not having the amount of food it is accustomed to. I would guess this might mean that your diet is too extreme.

If I understand correctly, any real weight loss is from the quantity of fat you lose.

I am pretty confident that getting rid of fat consists solely of using more calories (through activity) than you eat. I doubt that eating less food would increase the amount of fat you have... although it is also possible that your body may lower your metabolism to save food when your diet makes it think it needs to save energy.

That could be how it started. It's true that if you starve yourself for a period of time, you are more likely to gain weight by eating normally later -- your body is in "hoard" mode, trying to have enough backup for the next starvation period.

This is the central concept behind yo-yo dieting -- every time you lose weight by not eating enough (as opposed to exercise), it's harder to keep the weight down and easier to put it on.