I, too, have had similar problems... my family and friends have told me that I look healthy, that I should work to maintain, not to lose more or to gain more... except that I, instead of thinking I'm too thin, still think I'm too fat. I'm 5'5" and 127 pounds, and while my WW goal weight was originally 130 pounds, I changed it to 125 because I wasn't happy with how I looked at 130. However, when I reached 125 at the night of my prom, I STILL didn't feel thin enough (even though my best friend later told me that she thought I was too skinny then). Because my body proportions aren't typical (I have big thighs and butt, with a very small waist), if I tried to diet until my thighs were as small as I want them, I would be positively anorexic looking on top (already, at 125 my bones stuck out a bit too much in the shoulders and hips). This caused me to obsess about the difference of just a few pounds... was I too fat? Was I too thin? Should I maintain?
In the end, I just realized that this is due to a distorted body image, and I believe that your issues about feeling too thin probably stem from the same thing. During our respective weight loss journeys, we have spent long periods of time being "imperfect;" we didn't feel beautiful, some of us (myself included) were consistently TOLD that we were not beautiful... this kind of emotional abuse that we subjected ourselves to every time we looked in the mirror doesn't just melt away the way pounds do on a diet. I don't know if everyone felt the same way when they were overweight, but I felt hideous, and it took me a long time for my psyche to catch up with my body, because even after I lost the weight I was still too embarrassed to wear short skirts (even though I now wear a size 2 or 4 in skirts) and still cannot be persuaded to wear shorts. In fact, after I first lost the weight, I was walking downtown with my mom and was passed by an overweight woman in shorts who I thought had a similar body to mine. After she had passed, I asked my mom if she thought I looked like that, and she was stunned. "That woman must be at least 300 pounds, Sarah," she told me.
So the bottom line is, many of us still have warped body images, especially if our experiences as an overweight person were particularly traumatic. If you are a healthy BMI for your height, if your doctor thinks you are a healthy weight, and most importantly, if you FEEL healthy and feel that you are capable of performing any activity you would want to perform, than you are the perfect weight. Personally, I have found that I am overly critical of what I see in the mirror, and I try not to make my weight loss decisions based solely on that.
Another tip for you... try looking at yourself in the mirror with your hand covering the image of your face, or look at pictures with your thumb over your face. Now pretend that this is someone else's body. How would you feel about that person's body? Sometimes it helps to try to remove yourself from the situation... when it's not YOUR body, you tend to be much less critical. If you look at the picture without thinking about the personal stuff you attach to your body image and you think, "wow, she has a nice body"... well then, you have your answer.
Sorry this post was so long. Hope it helped!
PS- If you'd like to check your Body Mass Index, you can do so here:
http://www.cdc.gov/nccdphp/dnpa/bmi/calc-bmi.htm
Deciding to gain weight
I can't tell if my bathroom mirror is distorting things, or if I really am too skinny. 
