There are many possibilities here:
First of all her inflamed pancreas which possibly means malfunctioning regulation of insulin and glucagon (hormones which have a great influence on glucose homoeostasis and therefore our weight). Cortisol levels from her pain might be a possibility, as well as those known side effects of opiates on digestion.
There have been recent studies about the correlation between opioid receptors and energy metabolism. If certain receptors are stimulated the body can become less efficient in the regulation of fatty acid oxidation.
Another study suggests that opiates may cause people to have a higher preference for fatty and sugary foods and also cause them to overeat.
Although I usually advocate the unweighed, unmeasured approach, in your GF's case maybe a food and workout journal might be helpful just to get a feeling for her macro nutrient intake ratio, kcal expenditure, rest and recovery. If cortisol is a factor cutting down on metcon and shifting more to strength training + mobility work might also be helpful. She might have to lower her carb intake and even adjust her fat intake accordingly to counter the possible weight gaining effects from her pancreatitis and/or her medication.
The main goal is to get the pancreas back on track again. I don't know how strict her diet is, but she might have to go very strict paleo (gluten free, no legumes, no dairy, no nuts, no seeds, no eggs etc) or might want to consider to try the GAPS diet as once mentioned by Chris Kresser.