I have also been underweight most of my life and at 33 feel much healthier. This is what I did. Sounds gross but it helped: Coffee enema... Use organic coffee and a clean enema bag. Since you have a lot of gas, I am assuming you are filled with impacted fecal matter. DO that once a week. Also, drink apple cider vinegar (1 tbspoon of Braggs)through a straw once a day with a cup of filtered water. This will balance your ph. Another health rememdy I do to rebuild my good bacteria is one raw egg yolk (certfied organic) one lime juice and olive oil to stimulate bile in the liver. It sounds to me like your liver is congested. This all sounds CRAZY but it worked for me. If you only do one thing, do the Apple Cider Vinegar because that will help you a lot but drink it through a straw so it doesn't ruin your teeth. Let me know if you have any additional questions.