logically it's the incense. but i've been to many spiritual stores, there's an energy about them, some can be more intense than others, especially if they preform rituals in the building as well. don't worry though as they're a positive space and most who work in these types of stores can feel the energy and would banish anything negative.
my store actually has a singing bowl they use to cleanse most objects so you don't have to, but you can cleanse the items yourself at home in a number of ways. simple one is to use salt water. you might also consider getting into a routine of energy work yourself as negative energy clings to use throughout our daily lives. sage can cleanse, as can a sea salt bath, or grounding [among other methods] so try various methods and find one that works for you. the more you work with energy the better you'll be feeling and dealing with it.