Normal body fat % for women is 25-31%. Of course, for very slim women or athletes, this will be lower. But still, under 15% is not considered healthy- as you are finding out- your body can't support itself.
I would push the doc about it- especially if you're not getting your period, that's a sign that your body isn't operating as healthy as it should.
Healthy ways to gain weight are just increasing calories. You should consume about 300-500 more calories than you burn if you want to gain weight. So, depending on how active you are, you will need to adjust. There are lots of good websites to help you figure all that out- if you pm me I can send some to you.
Lots of people have reported success with nutrient shakes and that type of thing- high nutrition, high calorie shakes and bars and such. Plus, just eating enough!