Activity update?

It is possible to adjust the goals on your phone - scroll down or use the crown wheel to get to the bottom of the fitness screen, then tap the Change Goals button but I've never seen it adjust on its own.

Given the multi-step process to changing it, it's unlikely that this happened accidentally. Maybe someone else in your household used your watch?

The only way for them to change is for you to do it on your watch.

Change them back to what you want them to be.

From that same screen scroll down to the bottom and tap change goals