Healthy weight gain? If you're already healthy, it's probably not going to happen, sorry. Stay hydrated and eat more foods high in healthy fats to take in more calories maybe. You've probably lost quite a bit of weight through water loss if you've been working out.

A few of the suggestions made by other forum members are going to backfire. Having more protein will regulate insulin levels and keep your appetite under control, and so will most fats. Putting on muscle mass is going to speed up your metabolism, and you won't bulk up like a guy. Worry about your health, not your weight please.

You could always mess with certain hormones by skipping breakfast in favour of coca-cola, and eating greasier foods for dinner and munch more cookies and noodles, but that will get your from skinny with high metablosim to tired fat slob in no time.