Losing weight in a bad way???

Towards the end of last year I quit my main vice (Pot smoking). Ever since then I have been losing weight like crazy, going from 155 pounds to my current weight of 137 (I guess not eating a 3rd dinner at 2 A.M. makes you lose weight, who knew?). According to my BMI, as an adult male of 5'9 I am still healthy, but BMI is a pretty ineffective if you aren't extremely fat or skinny. However, isn't 137 a little low? If so, what are the best foods that I should eat to best control my weight gain? What weight do you recommend I bulk up to? I will be hitting the gym soon so I would like to know.