I am having the same problem. I had different reasons for eating so little, but now that I am trying to increase it I feel so sick. I am trying to eat a similar amount but do it more often. The problem is that even just a container of greek yogurt fills me up for several hours. I could switch back to regular yogurt, but I don't want to eat foods that aren't as good for me just to eat more. I had a day last week where I ate 1400 calories after several days at 800 and it was a disaster. The next day I had trouble getting 600 calories in.
So, I think the advice above is good - just try to incorporate the extra calories gradually. I have had some greek yogurt and a packet of oatmeal today for my breakfast and snack and even though it's close to lunchtime I still feel nauseated from eating that much.
What scared me a little was that the day that I ate more, I felt so "ugh" and fat and bloated. I had a terrible workout and I was up a pound on the scale the next day. I know it's the increased food weight, but I am on a horrible plateau right now (I am sure from not eating enough) and seeing the scale go the other way is hard. Mostly, though, what scared me was my reaction to the increased calories. I felt "out of control" and like my eating was scarily similar to the eating that got me overweight in the first place. I know that's irrational and disorded thinking and that's why it freaked me out.
Again, I really think introducing those extra calories slowly is the way to go. One day at a time - just add something in each day and then the next week try to add something else.
HW: 225, lost 75lbs in 2011 LW: 150
Losing again starting from 205lbs - 7/31/2016
No words of widsom. After a lifetime of reflection and dieting, I still don't have this thing figured out. I have decided, though, that sugar is the devil and I'm cutting it out as much as I can.