Definitely have a vet check her out. A few months after I got my horse he got an edema in his girth area and it turned out to be blood worms so that my be another reason for the swelling besides for allergies.
If you think a product you may be using is the cause of any of the symptoms stop using until your able to get it cleaned if possible if not replace it. If it is able to be cleaned use products that will kill bacteria and an anti fungal. Best way to make sure those products wont cause a reaction on her is to use whatever normal shampoo you use on her to clean unless you suspect that that may be a cause as well so if you work with your vet you will be able to rule things out to find the right course of action.
I know you said you dont share your stuff with anyone but is it possible they are using it without your knowledge? There are just some people you make it common practice to “borrow” other peoples items without asking. Since I dont know how your barn is set up maybe start keeping whatever tack you can in your tack box and lock it if not then in your car so you know for sure no one else is using it