I don’t remember my first period but it was just normal I think. You being underweight may actually be something interfering with your periods but don’t think too much of it, just eat and stay healthy! I’ve had major hormonal disbalance since sixth grade, I had excessive bleeding which led to anemia at one point lol, but I got no cramps! Recently, I’ve had hellish cramps but minimal bleeding, see the pattern here? When you bleed too much or the blood flow is more, the cramps subside! which is why chocolates are a staple relief from cramps! Now, you got cramps right? So that may be why you had less or minute bleeding, but just see to it next month because it is your first period! 💗