Hmm, idk about not very pleasant though. Do you have a thermometer? What temp is it at, how long are you in there at max temp, and do you sit on the highest bench where it is hottest?
I always get to a point where I am like "man I gotta gtfo of here this is crazy". You just feel like uncomfortable, perhaps slightly physically anxious (antsy, almost jittery, like you wanna get up and move right fucking now). That feeling continues to get worse and worse until you just are almost forced to get up and leave. That is the dynorphin hitting you all at once then continuing to rise.