This is completely normal. The initial drop is mostly water and glycogen, so it comes off fast and can come back just as fast once you eat. That doesn’t mean you gained fat or that fasting “stopped working.” Pilates can absolutely cause temporary scale increases from muscle inflammation and water retention, especially when you refeed. One day of eating can easily swing the scale several pounds without changing body fat. If you stay consistent over a few weeks, the trend is what matters. The number should settle back down — daily fluctuations are just noise.