You've received a lot of good feedback so far. before going and getting checked out you may want to try to tweak of few things in your diet.
Make sure you are drinking 6-8 glasses of water a day, 4-5 servings of fruits and veggies a day, watch your carbs, eat whole grains, and watch your portion control. Keeping a food diary really helps you see exactly what you are eating.
Chances are you are not eating enough. If you are exercising and not eating enough and drinking water, your body goes into starvation mode and it will hold on to the fat. (Especially belly fat.)
I was in your situation, I had slowly gained 50+ pounds over 15 or so years. I tried everything, but until Weight Watchers nothing worked. Since Weight Watchers is a life style change and not a diet, it worked great for me and I have been steadily losing 1-2 lbs per week for 6 mos. (And I don't exercise except once or twice a week!!/
Yes exercising regularly builds muscle, muscle is more dense so you may notice your clothes fit better or you are losing inches but the number on the scale isn't moving. So definitely use your clothes as a gauge because you won't lose lbs every week but as long as you notice inches you know you are on track.
You might want to consider Weight Watchers to help you in your weight loss efforts. I joined online and I do not attend meetings. Their web site has a ton if info and if you are diligent using only the online tools it will work!
If all that doesn't work, see your doctor!!