Short term yo-yo effect (the one that happens in the day following a fast) provided the calorie intake is not hypercaloric, will not result in fat.
Long term yo-yo effect, when you lose a lot of weight fast and then go back to old habits or overcompensate with a hypercaloric diet will result in fat gain.
In your case, you may gain some water weight and glycogen which is normal, but any fat lost will stay gone if you don’t eat at a surplus.
Edit: punctuation.