If you are having dietary problems, I'd STRONGLY suggest you to check out the VEGAN lifestyle!
For DECADES I'd suffered with usual bouts of flatulence, sour bms, bloating, pain, liquid semi-soft to liquid stools, etc. (nothing bloody & never had a hemorrhoid.).
I'd been tested for everything but nothing came back positive. I don't have sensitivities, or allergies. I decided to do something radical for my health & to TAKE CONTROL!
I studied, studied, studied about veganism (a step above vegetarian & a step below a raw foodie) & found out what I should serve my family per day to fulfill dietary requirements, it's a BREEZE! It's just another way of life.
Since getting off ALL animal products (meats, dairy, eggs) which are DISGUSTINGLY RAISED & TREATED), reducing to oils & refined sugars/flours, which includes MOST semi-packaged & prepared foods!. I have absolutely normal, regular bowel movements each day. I never hurt, I never have bloating, indigestion, heartburn, upset/irritated stomach, intestines OR bowels!
Now, I don't think it's radical at all to eat LOTS of fiber-rich foods, fresh, vast variety of vegetables, fruits, beans/peas, grains, nuts, seeds! Anything we NEED except B12 can be found through the wh0lesome nAtural, plant-based f00ds! I follow the recommended daily requirements. I no longer eat any of my vitamins, minerals, protein, or calcium SECOND HAND through an animal! I eat mine FIRST HAND!
Do the research. You CAN do it! SOOO many ills & dis-eases can be alleviated & even eradicated! GO FOR IT!
Were you able to find anything else out about this condition? I am in a very similar situation.
Any sort of carbonation can cause bloating. One thing you can try (I assume you are not willing to give up the diet coke) is to open the drink and put it in a cup and leave it in your fridge overnight so that there is no carbonation left. Other tests that you should probably have if the doctor thought you had giardia are things like H. Pylori.