You should NEVER heat cotton-anything in a microwave oven - it is always likely to catch fire. The problem has nothing to do with your microwave oven, it can happen in any microwave oven. The reason has to do with the way microwave energy interacts with cellulose-based materials such as cotton. Many of these materials don't absorb microwaves very well on their own when they are very dry and at room temperature. However, since they are likely to contain water, they will get hot, and once they do the cotton fibers begin to absorb microwaves extremely well. In fact so well that they begin heating faster and faster the hotter they get, with the result that the cotton catches fire. The same phenomenon can occur with paper, wood, and other common materials. Another material you might use is nylon and that should NEVER be used in a microwave oven because that is likely to melt.
I'm a microwave scientist and this is well known physics.