Allergic to wax tarts?

For the last year and a half I've been using wax tarts in warmers to make my house smell good. They've worked perfectly fine up until a month or so ago when I realized my allergies start acting up really bad when I melt them. I only have seasonal allergies, which are completely under control. I don't have any problem except when I melt my wax. I thought the scents used in them were artificial. What is in them that's causing my allergies to bother me?