There are a number of people here who get chest pains when their gas gets bad. It can even manifest itself in the shoulder blades and back. It could be from excess gas in your stomach or esophagus, or it could be radiating up from your intestines. If your heart tests came back clear, then I wouldn't worry about the heart, but rather treat the gas. Definitely try an anti-gas medicine--since you have such bad gas you may need more than the regular dosage if your doctor or chemist thinks that's safe to take.
Gas will definitely move on you and make you feel like you have a small bird in your intestines trying to fly its way out. You should never, ever eat anything that's sugar free; sugar alcohols are EVIL for people with IBS; 1 gram of that will make my intestines move around so much I think they are going to walk out of me and set up residence in the next postal code. And manufactuer's are tricky with it now; it's in EVERYTHING from sugar free gum and breathmints, to sugar-free cookies, low-sugar or no-sugar added ice cream and even in some low-sugar diet products (Slim Fast Optima). I don't know about outside the US, but in the US sugar alcohols are always listed on the Nutrition Facts table, right where regular sugar would be listed. There have been people on here that have dropped their sugar free products and have suddenly been free of IBS.
IBS is not caused by stress, but stress will definitely make it worse. You need to find something that will help relax you--meditation, yoga, Valium, something--so that you can feel some better. Because if you keep freaking out, you will cause yourself a panic attack, and then you really will have heart palpitations and shortness of breath, chest pains, etc. Practice some deep breathing when you feel your stress start to rise. You may even try and encourage your BF to do yoga or whatever with you. Tell him it will help you calm down and not feel so bad; that way he will feel like you are trying to do something about your condition and he will be helping too. That may ease some of the problems in your relationship. A lot of times men feel frustrated when "their woman" is sick and they can't do anything about it, so they get angry and, unfortunately, lash out at her sometimes. So if you make him feel like he's helping, hopefully he won't feel so helpless and angry.
And for your loose stools, try some calcium supplements. Lil will have to give you the link to Inape's post on how to pick a constipating calcium supplement since you probably don't have Caltrate 600 w/ Vitamin D in your country (probably named something else). You just have to get a brand without too much magnesium so that it will be mildly constipating, which should help form your loose stools. Calcium is also good about soothing inflammations in the intestines and healing damage caused by various stomach "bugs" and bad bacteria and excess stomach acids (bile).
You could also try taking some probiotics for a while and see if they help relieve your gas; gas seems to be a common symptom of bacteria overgrowth. There are a lot of posts on here that describe how probiotics work and why you need them.