90 pounds would be at the bottom of your normal BMI range. 115 is closer to the top of the normal range for your height.
I would say, eat the 1400 calories. Make sure they are nutritionally sound calories, obviously. Don't add in a donut!
More chicken, more complex carbs here and there, a slightly larger portion of nuts... You get the idea. Let the scale be your guide in this, not your perception of whether you "need" the calories. After being at the low end so long, you've probably just gotten used to that.
Jay