Staravation mode. Should I start eating more and does it eff

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I'm quite concerned with this 'starvation mode' that pops up mainly everytime I put in my calories for the day. I know it has been blogged or posted a million times, but has anyone(having similar issues) tried to reverse the situation? Honestly, I'm not too concerned about loosing weight. I set a goal, but according to my BMI I'm healthy and if I were to lose about 15 pounds would be underweight. What concerns me though is that this starvation mode is effecting the toning process. It is an equal worry that if I try to eat more it will be a big bust and I will gain weight. I feel it isn't that I'm eating too less, just everything I eat is usually healthy and few calories. I've never felt I was starving myself, but when I eat I stick to, 'If i'm no longer hungry stop stuffing your face'. Does anyone know what I should do? I'm twenty; have a healthy diet(drinking mainly water as well) and go to the gym 3-4 times a week (and if not that exercise for at least an hour at home.) I've been under by 400-700 calories. Please help :(

~Brandy