It´s absolutely possible that it´s just natural fluctuation due to salt intake, hormonal variation, exercise, having eat more volume yesterday and not had a proper bowel movement... No real way to tell after two days. At the same time: if you don´t accurately track your calories it´s also impossible to know if you´re regaining fat. What I can say is that you´d have to have overeaten by almost 4000 kcal in two days for it to be pure fat gain.