I hate to be a wet blanket, but I lived in an apt for 14 years downstairs, and I swear, sometimes it sounded like the people above us were BOWLING! And I always tried to be nice about it and mention to them when I saw them about the noise, but a lot of times it would be the same case as what you have; they weren't doing anything (that would account for that amount of noise! ).So I don't know what it is; maybe these places are haunted by past tenents or something
Or it's just really really bad insulation between upstairs and downstairs and everything vibrates or echoes or is magnified somehow. I don't know the answer other than moving. But I can vouch for the fact that sometimes living downstairs is not the greatest thing. It's a shame that she's being such a witch about it though. Good luck; it's always tough living in such close proximity to other people that you have little in common with. All you can do is try to be as considerate as possible and if she continues to complain, tell her to drop dead; you're doing everything humanly possible to appease her
