TTC and considered underweight

So maybe I'm trying to justify why I haven't been able to conceive after my miscarriage 5 months ago, but Is being underweight a real issue? I'm 5'6" and 105lbs. I've been small my whole life and consider myself health. I eat well and plenty. The most I've ever weighed was 112lbs and that was when I was on the pill. I don't go to the gym but I walk an average of 3-5 miles a day in heels with my job. Is being underweight an issue? Does anyone else have trouble gaining weight?