I'm the only person of colour and an American in the whole building. I sure hope that ain't why these people are acting like that.
I wish I could say no, that's got nothing to do with it. But I'm sure you know, far better than I do, that it'd be wishful thinking. Even though the race issue has a different dynamic here, it's still very much an issue.
So, yeah, it might be just because of your race. It might be just because you're American, or just because you've got a 'funny' accent. It might be just because one of them fancies your husband. It might be for any number of stupid, petty reasons, but the key thing is that
none of them is your fault. If you had done anything to even
remotely earn that kind of treatment, you'd know exactly what it was.
Some people are just awful, and none are more awful that 'mean girls'; it sounds like you live next to a few of them. I'd say the best thing you can do is remember it's them, not you. Don't bother trying to make friends with them; even if you could, at this point, would you want friends like that? (If you're not sure, watch literally any teen movie ever.)
If you're looking to connect and make friends with people, you're much better off looking for groups/activities related to your interests (i.e. a book group, dance class, sports clubs, activist groups, community volunteering, etc.). I think that's pretty much universally true once you're a 'grown up,' but especially true when you're so far from home.
Since Dundee is a university town, and has, as I understand it, a pretty up-and-coming cultural scene, there's bound to be at least one thing there you can get interested in. If you're brave enough to jump in, you might even find a whole new interest. And, bonus-- since nobody knows you, and you don't have to worry what they'll think, you could even try something really ridiculous and cheesy (e.g. square dancing, synchronised swimming) that you wouldn't be caught dead doing at home, but that you always secretly thought looked like fun.
In the mean time, please resist any urge you might have to try to change yourself in order to 'fit in' or be more 'acceptable' to people here. There's no need, and it never works, anyway. Remember, 'those who matter don't mind, and those who mind don't matter.'
But, equally, in order to preserve your own sanity, try to avoid taking things personally. Like the stomping on the ceiling? It could be deliberate, but it could also be coincidental. Because they've made you feel self-concious, and unwelcome, you might be overly-aware of it when it happens to you, but not even notice if it's happening to your husband. If you're able to just shrug it off and think 'what a clumsy cow,' you'll feel better.
(But, if you're anything like me, and find you can't ignore it like that, then you might want to actually start logging it. Seriously, make a note of every time it happens and any relevant details. Pay extra-close attention when your husband is home, as well. Looking at the hard data will most likely show you that it's completely random, and nothing to do with you. Or, you might find a 'pattern' that, again, has nothing to do with you; maybe she just always does her step aerobics on a Tuesday afternoon. And, in the worst-case scenario, if she does appear to be targeting you, you've got something you can use if you want to make a complaint to the landlord/police.)
Hang in there!