When I was on a low-fat diet and complaining of sudden constipation, my mother, a nurse, informed me that a low-fat diet causes constipation. So upping your fats may help both your weight and your constipation.
Are there any healthy fats that you can eat? Can you tolerate walnut, grapeseed and/or olive oil? All are high in healthy fats and calories and you can cook meats and veggies in them. I was just reading in a book on the French diet--which also mentions the Mediterranean one--that in some places in the Mediterranean, they actually take a tablespoon of straight olive oil as a form of daily medicine! If you can do the oil and bread both, get some spices and dip your bread into it one or two meals a day.
All nuts are good fats (except coconut, which is saturated fat, like found in meat), and you can also get peanut or almond butter (with or without added sugar).
Can you do dairy at all? Fat from whole milk, cream, cheeses (watch out, though, cheese usually causes more consipation), ice cream, and yogurt all are considered good fats (to the French, at least).
If you can, try to eat dark meat poultry, as it contains more fat.
Definitely stay away from protein shakes and fake junk like that; we've had people come on here who actually develop IBS after getting on that stuff.
My mother also has fibro. One thing I was reading the other day is that when some people diagnosed with fibro quit consuming aspartame (Equal), their symptoms disappeared. So no artifical sugars for you, of any kind. My mother had a fibro doctor tell her to eat organic food, as there was something either in the dyes, preservatives, artifical ingredients, hormones or pesticides in regular food that seemed to aggrevate the fibro. So she switched as best she could and indeed is better. It's not totally gone, but she's not been laid up in bed unable to get out for a couple of years now.
And lastly, while it may seem counter-intuitive, given that you want to gain weight, walk daily. Several people with C on here swear by a daily walk/run for their constipation. Yoga can also bring some relief (especially from gas), as can belly massage (I'm sure gutatasrophe will be on here soon to send you a link to where he learned how to do it; swears it helps him go).