The reason diphenhydramine Is associated with a slight increase in dementia risk is the anticholinergic properties. This risk increase is from correlational studies on other medications with anticholinergic medicines being taken for many years by elderly people, so the risk of taking Benadryl occasionally is low to none.
The prescription alternatives like doxepin have the same antihistamine properties without the anticholinergic properties when used at the prescribed dose.
This is an example where people can get themselves into the wrong outcome by assuming anything their doctor prescribes is a last resort quick fix, but anything they can source by themself is safer and superior.
It’s not a good idea to take Benadryl for many years, but occasional use or even for months while going through a difficult period isn’t going to cause dementia.