You don't give much information to work with, which doesn't matter, because this happened within the first week of you owning it. It was already sickly or dying when you bought it. Even if you overwatered it, it should be able to handle one or more of those spaced apart before it should croak. You should return it to the place you bought it. It is their responsibility to sell healthy plants. You should expect they will either refund or exchange the plant for another. Before you leave make sure you ask about how to care for the plant end up with.
It could be that the place you kept them is not very warm. I've gotten flowers before and the place I kept them was cold and within about 4 days, they wilted. Even with all the fertilizer we put.
The soil could have little nutrition. You need a Ph. soil tester. This is how to make it at home.
It also depends on how much you water them. Wild flowers get watered by dew, and sometimes, rain.