Healthy woman suddenly suffers: severe stomach pain/loud intestines/food intollerant

I don't know why this happened but it's changing my life badly. Jan 25th I was feeling fine. I'm 43. COPD but a non-smoker in very good shape. Had started a 3 on 1 off, six day week at the gym in November. Was VERY fit for 2 months, until late Jan. I started to feel tired. I began to get a yellow thin stool.
That week I felt much worse when I had a whiskey on ice at a friend's house. My stomach immediately burned, very pronounced loud gurgling and groaning from all of my intestines. I couldn't believe the noise. I drank 2 litres of water slowly. I then ate some soup to calm my stomach for the night.
Next day I felt very tired and did not go to the gym for the first time in months. I was having loose thin yellow stool 2 or 3 times a day. I thought I was getting a bit of food poisoning. I thought it was spinach, as my diet mainly consisted of beef, spinach, broccolli, and a rice/pasta. I ate very simple foods. Now I was sick and getting more ill by the week.
For 3 weeks I've been sick with the same symptoms: loose yellow porous stool, stomach hurts when food is eaten, stabbing pains across the upper abdomen when eating, lots and lots of very smelly gas. I only ate soup and water and crackers for these 3 weeks.
Doctors have tested me for parasites and now C Diff. What else could I have???? As my stomach aches...
Now in week 4.
I eat: probiotic yogurt. A probiotic drink once a day, a multi-vitamin, soup, water, tea, crackers, honey, tea.
I can now eat bananas. Some eggs. A piece of toast.
NO: Meat, Dairy, Sauces, Processed Foods, Mixes, Nuts, No Sugars.
My stool is getting better formed. A bit darker. That's good. That's the only good change in almost a month!
I'm hungry and losing weight fast. I've lost 8 lbs in 2 weeks. I've never lost or gained weight like that in my life. I'm generally healthy, a non drinker for 25 yrs, until the odd glass of liquor this fall. I don't have any symptoms of ulcers, heartburn, etc and never have. I've been able to eat anything all of my life.
Now I have an almost constant headache, a very sore painful stomach, upper abdominal pains, noisy intestines, bad smelly gas.
I am very weak. That really concerns me.
Is there any ideas out there? What tests should I also run that may be the answer to my sudden negative health problems???
Thank you.