well, first of all. nausea.. it AIN'T a drug... its symptom
to quote from the news source i read "Sources tell TMZ that Selena had a bad headache and nausea while on the show and after it."
so let me clarify again, A SYMPTOM.
my next point, i'm not selena and niether am i a friend or a close friend of hers, just a fan. so as much as i wanna be 100% sure that she doesn't do drugs (which i'm pretty sure she does not) i cant totally tell you for sure she doesn't do drugs. but to be honest, i dont think she does. (this conclusion is based on interviews i watch with her and just the things she does in general, from what i learn from psych class, that not really things or behavior of a drug taker.)
next point, ( yes i know tons of points, just bare with me) there is no rumor of her taking drugs, i believe you misunderstood the article.
here is an article that tells you about her visit to the hospital (also i just type selena in to google and got this, do yeah may not be the best source):
link
me last and final point (yes i know thank god right, i'm finally done) she was hospitalized for food poisoning or so the article says, she has already been released.
NOTE: all this info, i mostly gathered from a 30s Google search,do i have no idea if this is true or not, or she really was hospitalized as from as far as i can see the website is not really reliable.