First if your body is not getting enough foods it can easily go into starvation mode and decide to hold or even gain weight. 1200 calories seems very low, especially given how much you exercise. So maybe you should keep your food around 1300 at minimum? It's hard to know the "sweet" spot that will keep your body healthy while loosing weight, but if you don't find it you will have a really hard time loosing weight.
Second I think maybe you should talk to your dr about thyroid tests. The thyroid is vital in metabolism and if it's not functioning correctly you can gain or loose weight very fast. Talk to your dr about testing for Free T3 and T4 as well as the standard TSH test, because they are more reliable at showing what the thyroid is really doing.
Honestly, 30 mins of cardio isn't much. Are you just running or are you doing intervals? Are you using an incline or running flat?
I would recommend you add a weight lifting portion to your workouts and change up your cardio... the stair climber is the BEST machine for cardio in my opinion. Does your gym have classes? I would also recommend doing one or a couple in addition to your regular work outs.
Muscle confusion is proven sucessfull in aiding people to lose weight. If you do the same thing everytime you are not challenging yourself.
30 mins of cardio is something that will help you maintain your current weight - aim for 45 mins to an hour if this is all you're going to do. Also, work on intervals... run at your fastest 2 mins then walk for 1, etc. Experiment with the incline... run at a 2 incline... walk at a 5.
Also, you should never never never run on the treadmill flat.. this could lead to shin splints which will ground you from working out at all.