What possibilities could cause me to maintain weight (waivers only 5 pounds) but increase 3 dress sizes in 2 months?

Unclear: It is possible to gain fat while losing muscle and keep a similar weight. Muscle is much more dense (heavy) than fat so if you were exercising and have stopped this is a possible explanation. If you are having any other symptoms or if this is of great concern you should see a doctor for evaluation. There are a few potential serious causes of this type of weight shift that should be evaluated.