Well Carm, if Biscuit is anything like Harlie, Id say it is most likely a bite...lol. Harlie likes to chase the bees around the yard, trying to catch them in her mouth...which she has suceeded in doing a few times in the past couple years...you would think she would learn when they sting her in her mouth, or on her lips, but no, she will get her benedryl, then go right back to trying to eat another one...so if Biscuit likes to catch bugs in her mouth, it could be a bite of some sort...
But like you said, it could be from the treats too, just because normally you dont give her that many, and she isnt used to that much exposure to commercial food, so it could have had a reaction.
And if you checked the Halti, I would doubt it was that, just because my three wear them and even if you put them on tight, they dont really rub on their faces at all. With my three, the only place that it would possibly start to rub is on the top of their noses, since they are so smushed...but then again Harlies nose is like Biscuits, and it doesnt bother her nose at all since its longer. So I personally dont think it could be caused by the Halti, especially if you check it.
Id just watch it for a few days and if it doesnt go away, then have her checked out...I hope it disappears fast though!