I would like to try and answer your questions, hopefully I can help you somehow!
First cause, maybe overtraining, which causes the body to undergo adaptation.
Either, you can crank up your exercise, if you were doing 20 minutes of cardio per session, maybe you should increase that to 30-45min, as your body may has went into adaptation. However, going harder and harder everytime is not always the best thing to do, you should know when to cut back and relax a little, for sometimes overtraining might cause your body to adapt, so a good thing is to take a break 5-7 days off, do you change your workouts frequently??
if not, maybe you should, so your body wouldn't undergo adaptation.
Also, your calories might be too low "which I suspect may be the real reason for this plateau", and the only way to break this plateau caused by starvation is to EAT MORE. this happens when your caloric intake is very low for a long time, eat more of the good foods, go easy on the junk, maybe you should just increase your portions a little. this increase may need 1-3 days to take effect. most fitness models and bodybuilders follow this method "zig-zag method" they follow a low calorie diet (usually low in carbs) for about 3 days and then they kick it up with 1-3 days of higher caloric intake (higher carbs), this way you can trick your body before it figures out what is going on, and switches to starvation.
So therefore, ZIGZAG, 3 days low, then 1-3 days high. tryi this and I really hope for the best results!! believe me,it works 100%. If it doesn't check back with me. best of luck