I have heard of sufferers becoming 'manic' and really switching on to tasks and activities with a burst of energy - could be a delusional impulse triggered from past memory. If she is safe, then I guess it's good for the exercise; but she may not be as observant in the process and could come to some harm, so supervision may be in order. Perhaps if you talk to her at these times you might get some clue as to her intention (is it 'current' or using past references to people and things?). For all we know, she may just feel like doing stuff... and then need a day or so to recharge the battery.
Since you put your question under the dementia and Alzheimer's category, I'll assume your mother suffers from one of these issues. My mother is 94 with moderately advanced dementia and frequently tells me she has SO much to do that she has no spare time to herself. Which is not true at all, she lives in Memory Care Assisted Living and has zero chores or responsibilities whatsoever. She's just confused and feels compelled to be doing something. This doesn't happen all the time, either.....there are days she's bored with nothing to do, too. I never know which personality I'll be dealing with on any given day, it's always different. Such is the nature and misery of dementia.
Is there a reason why you are concerned about her working outside? I checked your profile, there is no information there. Some days I feel like working outside and some not so much.
For all we know, she may just feel like doing stuff... and then need a day or so to recharge the battery.