Any change in your feeding schedule, really, can lead to plugged ducts, which can later lead to mastitis. When you combine plugged ducts with the stresses of returning to work and leaving your baby at home/daycare, and a potentially low immune system from exhaustion, you have a pretty solid recipe for mastitis.
I thought that with my third child, I'd be somewhat immune from things like mastitis, which I never had with my first two babies. Clearly, I thought wrong! My first bout of mastitis struck when I went back to work (which literally meant leaving Huckie upstairs at our old Jane street location with his Papa for a couple of hours at a time). The second struck when I recently changed my work schedule, with a baby who is almost 17 months old!
Wondering about prevention and treatment? Here is a fabulous article from Kelly Mom that will help you figure out if you have plugged ducts or mastitis, and what to do about both! http://kellymom.com/bf/concerns/mother/mastitis/