How do I stop my kittens from having accidents outside of the litterbox?

I have 2 kittens, one is 14 weeks old and the other is 16 weeks old. I have had them for about 3 weeks now. They were doing so well and were litter-trained without a problem. In the past few days they have been having little accidents all over the house (#1 and #2.) I clean the litterbox regularly and have thoroughly disinfected the areas of the "accidents." The litterbox is very accessible to them. What can I do to prevent future accidents?