Hi Jonah,
My doctors told me I had IBS also. IBS always seems to be the doctors cop out.
I am not entirely sure what symptons you have apart from the stomach pains but I, like, you was losing weight rapidly over a shorty space of time, passing stools at least 2-3 times a day (unusal for me) and suffered with a constant belly ache accompanied by a dull pain near one of my ovaries. I saw 2 doctors from the same surgery, and the hospital did blood tests x2, gave urine samples, had a vaginal transcan (in case anything was wrong with my ovaries/womb), an ultrasound (in case there was a problem with my stomach/lungs - to rule out everything). I pressed the doctors, seriously, I was there every week for about 3 months. I had a stool test for the second time and they found something called hekicobacter (hplori) some form of stomach bacteria. A course of 930mg x 3 a day for 5 days soon cleared the problem.
I know what is it like when your worried about something and you don't know what it is but sod the 'rolling eyes' of the doctor and persist they give you a full MOT. Good luck :)