Unless a person is obviously adding more calories to their diet OR they used to exercise regularly and suddenly stopped, there is no reason to think lowering calories or upping exercise is the solution.
There's SO many things that can be the source of this, many were mentioned ... hormonal flux ... BIG ONE HERE, hypothyroid issues, adrenal issues, blood Sugar issues, digestive issues, liver issues ... the list goes on and on.
Hope you are successful and finding the source of this and are able to fix it! I totally understand what you're going through! This is an excellent forum to help you sort it out!