Can Constant Cramps,bloating,and Diarrhea Be Symptoms Of Abdominal Or Intestinal Lymphoma?

Question: Should I be worried about abdominal/intestinal lymphoma? In October I experienced sudden onset of diarrhea/loose stools. CONVINCED that I had colon cancer (I'm a 26 year old healthy female, with no family history of ANY type of cancer), I was very depressed and had no appetite. I lost about 14lbs in 7 weeks - from a combination of not eating and diarrhea. When the diarrhea first started, it was accompanied by nausea and burping - which subsided after about a week and a half, but the diarrhea/loose stools lingered. Went to see my primary doc who did stool culture, O&P, H. Pylori, abdominal xray, CBC, and metabolic panel. All normal. Referred me to GI doc. He did colonoscopy and that was normal. Primary doc put me on amitriptyline 10mg to see if it would work. Within a week of starting the medication, my appetite was back, and my stool was normal - actually on the constipated side - which is normal for me. Also gained back 4lbs in a month. I'm wondering if I need to be concerned about some type of tumor/lymphoma in my abdomen. The symptoms seemed to come out of no where. My doctor told me this week to cut the pills in half (making it 5mg of amitriptyline) because I was on the constipated side. Well I've been doing that for 5 days now, and I'm experiencing cramps, bloating, and diarrhea again. My question is - If I had an abdominal lymphoma/tumor, would I have more symptoms/constant pain? Would my blood work be off? Is the amitriptyline masking something more serious? Doc said that since the 10mg amitriptyline worked, it's IBS. I'm wondering if I should be worried???