I have a lot of health issues including chronic pain, so I'm on quite a few meds, including pain meds and it's absolutely astonishing how medications or a change in medications can affect weight loss.

Often the effects are temporary, but even so, it can take weeks or even a month or two to get back to a regular pattern. It may be your "old" pattern, or it may be a new one.

Also, as already mentioned, injuries and even exercise can also cause increases in water retention, because extra water is needed and utilized for muscle building and repair. Drinking a little more fluids can help in the long run, but can cause short-term stalls or occasionally even gains.

It's not "real" weight gain, and if you're patient it usually resolves in a few weeks, but it can be frustrating in the mean time.