The chances of you gaining a pound or more of fat are unlikely. You would need to eat an excess of 3,500 calories over maintenance to gain just one full pound of fat. Chances are you have a combination of more food in your body, and increased water retention from excess salt or hormonal issues.
IF YOU ARE USING MEASURING CUPS OR ANYTHING BESDIES A FOOD SCALE TO MEASURE YOUR FOOD, THIS WOULD BE THE FIRST AREA TO FIX. ALWAYS USE A FOOD SCALE AND GO BY THE GRAMS OF SERVINGS.
Your body can store water weight from having a higher salt diet, hormones, added stress, increased exercise, or many other reasons. The most accurate way to track your weight is to weigh yourself daily and keep track of WEEKLY AVERAGES which can paint a bigger picture of how your weight is trending.
Keep in mind that just one “cheat day” where you go over your calories by a lot is more than enough to wipe out an entire deficit for the week. It’s important to track ALL days, including cheat days.
If you are absolutely sure you are tracking properly, it has been a few weeks and you’re eating at least 1,200 calories per day – it is time to visit a doctor or a licensed medical professional for a checkup.