I use primarily cotton thread in all of my work and I have not had anything break - and the quilts have been through the wringer.
Do you still have some of the thread so you could do a tug test? You should not be able to tear off a piece of the 30-weight, at least not easily. I suspect that your quilt stitches might have been too small and that the tension was off.
Can you requilt it? If you don't want to see the second set of stitches, simply use a good quality polyester (NOT Nylon!!!) thread.