Does anyone else have the problem of losing too much weight eating paleo/primal?

Basically I was introduced to Whole 30, primal, and paleo early summer 2011. Whole 30 was recommended by my physical therapist at the time because I've been dealing with a strangely resistant shoulder repetitive strain issue (still persisting - over two years now) and he was confounded enough to see if my diet could be part of the issue. Eating differently than the SAD was new to me but I gave it a try.

Some quick background info about me - I'm 35, female, 5' 11". Always been very nonathletic and skinny with little to no muscle tone, what I call 'skinny fat'. I did not care about losing weight when I started the new way of eating, as weight was never an issue for me. I was mostly interested to see if it would help with my RSI, or my ever present fatigue, or my digestion issues ( frequent diarrhea and bloating). I grew up on a standard American diet, with the combination of some good food (mom had a vegetable garden) combined with lots of bad food (grains/beans/pasta, commercial meats, vegetable oils, lots of junk food and soda)

Initially I lost a good 5 pounds and my almost constant stomach bloat that had been around for last couple years completely went away. I've gone through varying versions of primal from then until now, sometimes including dairy sometimes not, symptoms including grains, sometimes not, as I wrestled with my own beliefs and doubts and theories and peer pressures and sugar addiction.

For the last couple months at least I've been doing pretty much strict paleo, thinking that inflammation is probably a huge part of my problem regarding the RSI and half convinced that I might be celiac, though Dr. was not interested in proving it. I increased my veggie intake (trying to get 6 to 9 cups a day), went out of my way to make sure I was getting organic eggs and organic grass fed meat, avoiding all processed food, and not eating out at all, and limiting my sugar is much as I can. My fruits are limited mostly to berries, though I definitely would sneak in a banana here there.

Annoyingly, this new healthy way of eating is not really improved my constant fatigue or diarrhea issues. But more important than that, I really cannot seem to stop losing weight. Initially before I started I was about 140 lbs. For my height that was fine, though the muscle fat ratio was not good. Since the initial 5 lb loss I've been losing steadily an additional a pound or two every month regardless of how I vary my diet (as long as it's still minus grains). Every time I think it's as low as it should go, and I hope that it plateaus, it just keeps getting lower. Right now I'm down to 119. I was very concerned around 125, and I don't understand why it just keeps going down. at 119 my BMI is 16.6. This is definitely underweight.. I think I weighed this much in junior high.

I've always had what seems to be a high metabolism (despite my fatigue and constant lack of energy), my body just never cared about storing fat. And now that I eat healthy I can't seem keep the weight on. I could start eating starches and grains again, but I'm distressed at the idea of doing that because I care about my health, and I don't think adding a bunch of bloated weight is going to really help my situation. my doctor wants me to gain 15 to 20 pounds. I have no idea how to do this without going back to starches. I'm pretty sure I get enough calories. A typical day is around 1500 - 1700. I've been adding shakes that usually include coconut milk, frozen berries and protein (hemp or Garden of Life brand protein) just to try to get some more calories in, but doesn't seem to matter. I've also tried eating more nuts of varying kinds, but they often cause digestion issues for me too - not fun.

I'd love to add some muscle, but my RSI prevents me from exercising my upper body. My glutes and abs could really use some tone.. But I'm doubtful that exercising them will gain me the kind of weight that I need. I started doing 30 min. of cardio 3-5 times a week a couple months ago to try to help with RSI, and Ive played around with lower body machines at the gym, but so far do not really see a difference in tone. I'm always a little confused as to how much weight I should be lifting when it comes to lower body, and what kind of reps and sets etc to actually build muscle, as exercise is not really something I've done seriously at all before.

I'm frustrated... I'm looking for any and all advice regarding ways to gain weight without compromising the health of my diet. thanks in advance.