Without question call your doctor and make an appointment regarding the tenderness in your belly. If you are still looking 5 months pregnant and have tenderness, it is possible you developed an umbilical hernia during this recent pregnancy. Irregardless, get it checked out.
In regards to loosing weight, the quickest and easiest way to loose weight and keep it off is through healthy heating habits. Try eating high protein/low fat foods. Eat smaller portions and eat at least 5 times a day. An example would be an egg white omlette with veggies (no cheese) for breakfast around 7 or 8 (whenever you are up and the kids are situated), a half cup of fat free cottage cheese around 10, a can of tuna (try mustard instead of mayo) with a cup of veggies for lunch around 12, a cup of yogurt at 3, a whole baked chicken breast seasoned to taste with a cup of veggies for dinner, and if you are still hungry before bed you can have another high protein snack before bed. Eat the veggies fresh or steamed, no butter or oil. Within 2 months you will see MAJOR results, particularly in your clothes, as long as you are strict about eating throughout the day. Obviously, if you want to tone however, you will need to exercise, but all you really need is about 30 min of resistive training 3 times a week. You can do it once the kids are in bed (pushups, tricep dips from a chair, squats, lunges etc). If you really stick to this you won't need to do much cardio if any at all, unless you want to strengthen your aerobic capacity.
Unfortunately loosing weight and toning up take tons of dedication but don't forget that you are worth it and you will feel so much better about yourself immediately just knowing you have started the process. Good luck!