Not sure if this is or isn't inflation. Can't read Japanese, so I'm just going off of the illustration, but so far as I can tell, she signs-up to test a new kind of drug or treatment... So far as I can tell form here, there are three possibilities: 1) The drug itself causes a major appetite increase (or perhaps she's naturally a big eater gorging herself on the money from participating in the trial), and she so much she gets constipated quickly 2) The drug induces some weird metabolic change that makes the body use all of the eaten rather than expelling it, hence she's not actually constipated but simply believes she is and that the excess girth comes from associated bloating, hence the enema & such (this is a more farfetched interpretation, but Katou's no stranger to that sort of thing, and frankly I like it best, plus it makes sense with the scientist at the end) 3) The drug causes severe constipation/bloating and Katou just puffed-up her tits & thighs when drawing her bloated.
I'm sure there are more, and you really can just follow your own headcannon, probably, but I figured I'd put those out there