Overtraining is a result of the damage you do to your body through exercise exceeding your ability to heal.
If you're doing Rip's program (I'm assuming Starting Strength) and you're new to lifting, you really aren't taxing your muscles that much. This is due to the fact that your neuromuscular connections just aren't efficient enough yet to work near your body's actual work capacity.
Swimming is a cardiovascular activity purely by the fact that you do it at a low enough intensity that you can sustain it for longer than the three minutes of the anaerobic metabolic cycle. You use more muscles than you do in other aerobic activities like running or biking, but it still doesn't tax your muscles that heavily.
Over training on cardio is very, very difficult to do. Over training is pretty prevalent in the Iron Man and marathon communities, but I have a feeling that your workload isn't nearly that high.
Eat and sleep well and you'll be just fine.