Dissatisfied customer

I have made a laybye @ Miladys of underwear, then on the 1st June I went to take the laybye out while I was busy with the cashier I wanted to exchange some undies and she told me it was not possible to exchange but my question is I know about hygiene, I didn't go with those undies @ home they reminded in the store, why was I not allowed to exchange them? I just want to know as a consumer what are my rights?