Are you eating real food now? Nutrisystem is very high in sodium, which may be part of the problem there. Try switching to lean meats, veggies (but skip the high starch/sugar ones like potatoes, carrots, sweet corn, peas), fruits (but sparingly with sugary juices, bananas, etc), and keep it lower on the carb/sugar intake. You'll have to do your own portion control, but can bulk up with things like fresh spinach if you feel a need for a big portion. Drink your water. Watch the energy drinks, which can be high in sodium as well.
Your workouts sound long, but possibly not very intense...? If you're walking 10 miles a day, 6 days a week, maybe it's time to switch it up to a running program? Are you lifting heavy to build muscle, or lifting light/high reps? You might do well to get with a personal trainer, at least to evaluate your workouts and give you a new program that will increase intensity and shake things up at least a few days a week. You're probably just fit enough now that your body isn't getting a lot from the workouts you're doing.
It can't hurt to see a doc, to rule out a medical cause, but just thought I'd toss those ideas out there for you. I'm sure you'll find a way to get things moving again!
Cheryl