Google "why the scale lies" for insight in to water weight fluctuations. I can't believe your doctor didn't tell you about muscle edema. When you increase/change your workout, your muscles retain water to help them heal. This can show on the scale as a temporary gain, no loss or smaller than usual loss. So you might see a gain on the scale but your pants get more loose. The scale eventually catches up. Muscle growth can also contribute, but it takes time to build muscle.