Toning up doesn't exist. Also starvation mode doesn't exist. They are both mainstream concepts that vary in definition and include a wide range of different ideas. Your metabolism is crawling right now though because you are severely under-eating and things will only get worse if you don't make sensible changes.
What you are trying to do is improve your body composition i.e Gain muscle; Lose fat. I don't know how severely underweight you are since I don't know your height, but what is clear is that:
You are underweight
Let that sink in a bit. So having established that it's clear you DO NOT need to lose weight. Instead you want to develop a physique, such as the gentleman below.
This guy looks like he's 5'7 and around 140-145lbs. So set that a goal similar to that.
Part of the reason why you think you look fat is because your muscle mass is so low. The picture below shows a male who is probably around 120lbs or less yet you can't see any Abdominal definition or definition in the rest of his body.
Here's the thing: The toned look is a combination of having high muscle mass AND low bodyfat. If you have low bodyfat AND low muscle mass you end up looking like a starved person.
What to do
Start increasing your caloric intake which will lead to your metabolism revving up. This will lead to better hormone production (Which means more fatloss/muscle gain), muscle gain, and muscle size increase (Your muscles will be filled with glycogen -fuel- which makes them bigger).
I recommend slowly ramping up your calories. Start by ramping up to maintenance. Keep that for 1-2 weeks and then you can add calories on top of that if you want to gain muscle faster.
Here's how you figure out your caloric maintenance level
- Calculate your BMR
- Multiply your BMR by an activity multiplier
- Add a caloric surplus (500-700 calories for example) to speed up muscle gain
A note about weight and increase calories
When you start eating at maintenance your body WILL gain weight. That's purely water weight, glycogen, and food in your stomach. It's temporary and essential. Think of it as your body putting up scaffolding to do some repairs. The water levels WILL go down within a week or two and you will end up looking significantly better. For a detailed explanation on this see my Water & Scale weight article.
Make sure to keep exercising and look into resistance training as that's the best way to gain muscle.