Have you been logging your calories into FitDay to get your calorie count? And check your nutrients levels?
It's good you're eating a healthful diet now.
I'm worried about you though. There is a very real risk in not consuming enough calories. You're very active if you're walking 90 min a day. If your body doesn't get the nutrients it needs to maintain itself from food, it takes them from elsewhere in your body (for example, calcium from your bones, other nutrients from your muscles). Many young women who don't eat enough develop osteoporosis as the body takes calcium from their bones.
Heart attacks are another possibility when the body isn't getting enough nutrients. The heart is a muscle and the body will start consuming it to get the nutrients it needs. A couple of years ago, a local 16-year-old girl who wasn't eating enough had a heart attack. She survived, but she was without oxygen to the brain for long enough that she became brain damaged and will require full-time care for the rest of her life. Several very thin models have dropped dead from heart attacks.
My suggestion would be to eat enough to get your BMI up to a normal level. See a doctor or nutritionist to give you a healthy diet plan. Remember that muscle weighs more than fat. When people exercise and replace fat with muscle, they frequently experience a weight gain.
Age 61, Height 5'9"
March 2010 145 lbs, Jan 2013 173 lbs
1st Goal 145 lbs 5/25
New goal 140