People can get upset by gaining 3lb or so in the space of a day. Based on SP guidelines, it would take an extra 10,500 calories to add 3lb of fat so it's probably water weight, which will come and go. Factors such as TOM or eating too much salt and sugar can cause this. Don't let it put you off.
Try reducing salt and sugar and eat foods with natural diuretic properties that can reduce fluid retention. These include asparagus, Brussels sprouts, cabbage, carrots, garlic, lettuce, tomatoes and oats.