I think he's probably referring to the rash that he has right above his "2Pac" tattoo on his chest in that one picture. I dont think it's ever been proven wat the deal was with that but it seems it was probably just a regular rash.
If you listen to the audio commentary on the Tupac Resurrection dvd, they do talk about this. The reaction in his skin was not stress related but, a result of the tatoo he got.
the commentary on that that i remember from resurrection dvd was his sister talking about during the sex abuse trial the nerfertitii 2 die 4 tattoo fizzin