The answer to your question is complicated. To say it short, yes, one can be healthy and underweight, but it is not your case. Some people's body structure just not fit to be underweight and some will just look a bit chubby. Most women are built to be a bit chubby, otherwise they could not hold a baby inside of them in a healthy way.
Just like Surnia said, you've done it before and it made you I'll, it has to show you something....