The Spaniard suffered a bruising weekend at Mugello, crashing heavily twice on Saturday – the second one leading to him hitting a barrier lining the track.
Qualifying 19th, Fernandez struggled to 17th in the race.
Having been unable to sleep much the night before the grand prix due to the pain from his crashes, Fernandez had been given medication by MotoGP doctors to help with this.
All of this, however, combined with the intense heat of the Mugello grand prix led to him being sick in his crash helmet – which left his visor "dirty" and led to him almost tangling with Tech3's Augusto Fernandez.
"Saturday was a very difficult day, I qualified very badly, but the truth is that this Sunday the pace was very good, I went out and I was with Maverick Vinales, with Miguel Oliveira, we were catching Aleix Espargaro," Fernandez said.
"Our goal, to be with the other Aprilias. However, at one point in the race, I was able to pass Fabio di Giannantonio and I went after Maverick, who had got away a bit.
"I caught up with him and then, I don't know why, whether because of heat stroke, because of yesterday's crashes, because I was taking medication and I hardly slept tonight, but I started to feel bad, bad to the point of vomiting on the bike and having to slow down a bit.
"I tried to hold on but I almost took Augusto with me when he passed me, I couldn't see anything inside the helmet. I was riding with a dirty visor."
Fernandez added that "I don't want to blame anyone, least of all the doctors" for his condition in the race, while admitting he "looked like Robocop" in the morning warm-up as he was so stiff on the bike.
Asked if pulling out of Sunday's race ahead of time was an option, he replied: "No, not at any time, not even as a joke.
"There are many people working behind us and it would be disrespectful.
"In the end we are paid to race and no matter how bad you are you have to go out and show your face, and that's what I did - show my face for the team and for Aprilia. Until it blows up.
