Himeno told Denji that if he killed the eternity devil in the building then she would give him a French kiss. This young man is extremely motivated by sexually gratifying rewards. He eventually kills the eternity devil. They all go out to a restaurant where mos of them drink heavily, the one of the exceptions being Denji because he is too young to legally drink. Himeno gets really drunk before following through with her promise and as a result throws up into his mouth a lot while Frenching him, he instinctively swallows all of it.

Side information: Denji inherited his dead fathers debt at a very young age, having to fend for himself and pay the debt alone. As a result he was very poor and would eat anything he could find that would give him calories. Things like moldy food from dumpsters. So even when the thing entering his mouth is vomit he has the knee jerk reaction to not waste it.

So after he swallows the vomit he then is in the next scene throwing up in the toilet. That night he is in bed really sick from the ordeal. The correct interpretation is cause he had been throwing up a lot. This person has suggested that it is cause he was secretly drinking alcohol cause that’s common amongst teens. However there is no explicit reason to believe that and it’s a fan theory at best. Considering that the writer clearly doesn’t shy from fucked up themes shown by all the sexual and gory content I don’t find it realistic that they would have Denji drinking alcohol in secret from the viewer. Since him throwing up in the stall is a valid and explicit explanation it makes much more sense to assume that is the reason he was dizzy that night.