Should I take on another class?

Stick with 13 credits. Lab classes tend to be a lot of work for a little credit. Plus you have a job and you need to figure out how to balance work and class. Worst case scenario of a light schedule is that your schedule is too easy and you get straight A's. Worst case scenario of a heavy schedule is that you get overwhelmed and do poorly in your classes. You can make up that extra class in a later semester or as a summer course.