Don't weigh yourself, it's an inaccurate way to determine "weight" loss. The only real way to measure results is to get your Body Fat % tested (with calipers, not electronic meters), and then have it done at intervals (every 2 weeks, for example). I could lose 10 Lbs overnight by simply making small changes to my diet. It'd mostly be water weight, though.
Right now you're not eating enough. For someone your size (having your BF% would help), I'd recommend starting at 2500 calories and then adjusting.
When you eat more, your body accommodates by increasing your metabolism. You should try to aim for 4 to 6 meals a day.
It also sounds like you're eating too little protein.
I'd bet you lost very little fat, that 15Lb was mostly water. No, that's not unhealthy, it's quite normal. Most experts agree that losing more than 2Lbs of fat per week is too much. You measure that with calipers though, not a scale
Of course the mirror test never lies... if you're slimming down, you're obviously losing fat (or a hell f a lot of muscle
)
Are you doing any weight training or cardio right now?