You are unclear about your numbers. You mentioned that you consume 700 calories a day. Then you mentioned that you have 500-700 calories in the morning by having oatmeal and banana. Does that mean that your consumption for the rest of the day is 0 - 200 calories?
To start with you have a very low calorie consumption. The recommended consumption for an average female is 2000 calories a day and for an average male is 2500 calories a day.
You might think that by consuming 700 calories a day only you will lose weight. This might seem a good idea to lose weight, but believe me it is not. Consuming too low calories will trigger the starvation response, which is your body's natural mechanism to keep you alive. By starvation mechanism the body will slow down metabolism so that it preserves the little energy that you have to support your vital organs, such as the heart, kidneys, lungs,...
Your low calorie intake is confirmed by the low energy levels that you have.
If you want to stay healthy and lose weight I recommend you do the following:
1- Increase you calorie intake to the normal levels I mentioned above. Your calories should come majorly from protein, complex carbohydrates (found in fruits and vegetables, and healthy fats.
2- Eat 5 to 6 meals a day (3 main meals and 2 snacks).
3- Decrease your consumption of sugary stuff as much as possible. The less the better.
4- Do 4 weight training sessions a week. Weight training build your muscles which is your best friend when it comes to weight loss.
5- Do HIIT exercises. High Intensity Interval training is a cardio training where you make short intense training bursts followed by slow intensity periods. This has shown to burn more calories than regular cardio training.
Good luck