Muscle toning when you're underweight?

So I'm a 19 year old girl with a BMI of about 16 (5'3" 90ish pounds). I've always been this underweight, but I'm not particularly fit. Which is why I recently started exercising. I want to basically tone my muscles and get rid of some belly fat so you can actually see the abs when they come. I'm basically doing pushups/semi-pullups/squats/core for about 20-30 minutes a day, and cycling for about 45 (mixed flat and mildly hilly terrain. Enough to get my heart rate up to 130-140 after 45 minutes). 

I can see some results already, because the muscles show fairly quickly when you haven't got a lot of fat, but I'm concerned about losing too much weight. How should I be compensating with my intake? Will my body automatically crave more calories/will I feel hungrier/should I not worry, or do I have to consciously change the type of food I eat? 

You cannot tone muscle (it is a sort of marketing word). We all have muscle tone and we all have a fat % and to have muscle tone show you must reduce body fat%, sadly for those already in a lower weight or underweight you must gain muscle to lose body fat which is harder and slower than an overweight or someone with weight to lose to lose fat. Naturally very thin people (ectomorph) tend to have a hard time gaining muscle so this may take more effort/time on your part.

Article: http://www.bodyrecomposition.com/fat-loss/red ucing-body-fat-percentage-by-gaining-muscle-q a.html

Basically eat at a surplus, lift heavy with full body workouts 3 days a week and 6-8+ weeks you should start to show fitness results full transformations can take 3-6 months to years. Eating better should be an added benefit (eat better preform better in workouts).

Add muscle.  Start with a program like this one or this one, and eat a lot.  

Toning isn't real.

If you want bigger muscles and more definition, follow smashley's advice.

Thank you! I'll try this out. Just goes to show how much of a newbie I am at this, haha. Oh well, all in the name of fitness!