5'6.5" here. Current weight: 125, but I have gained a couple of lbs since Thanksgiving (gotta start running again!), and I'm not underweight. I try to consume an average of 1900 calories per day. During the summer, when I bike 2-4 hours, I am careful to have a meal soon after the ride.

Protein helps me keep hunger away, for the most part: I added protein to my breakfast (hard boiled egg whites, smoked salmon, Greek yogurt, peanut butter... ) and at the office I keep a jar with a mix of almonds, pistachios and cashews, for quick snacks. A nutritionist suggested I increase my protein consumption to ~50g per day.

PS: At 5'7", you'd have to go under 118 lbs to be considered underweight by the BMI tables, but I think it depends on your overall bone structure. Some people can look emaciated at that weight, others look just fine.