What Could Cause Severe Stomach Pain, Rapid Weight Loss, Fatigue, And Frequent Bowel Changes?

Hi I have seen a few doctors relating to my severe stomach pains & rapid weight loss in the last few months. They have tested for things like ulcers, coeliac disease and gluten intolerance and they have all been negative. The pain is pretty constant & is much worse after I have eaten. Recently (2 or so weeks) I have been extremely fatigued some days struggling to even brush my teeth or put shoes on. I have dropped from 57-59 kgs down to 45-49kgs (this ranges week-to-week) in a matter of 2-3 months. My bowel movements are a lot more regular (2-3 times a day) and some times I feel like I need to go but can’t and then other times it’s really aggressive diarrhoea. I am worried that it is getting worse and is also effecting my kidneys as I have had 2 kidney infections in the last 6 months. I am only 23 years old so I’m very confused as to what is going on with my body but I cannot handle the pain anymore.