P!nk -- 'KISS MY ASS' ... I Didn't Fake Sick to Get Out of U.K. Concert

P!nk has a message to all the haters who claim she faked sick to get out of a performance in England -- "KINDLY KISS MY ASS!!"

Ever since P!nk pulled out of her April 22nd show in Birmingham, certain "fans" called BS on the singer's sick excuse ... and insinuated she doesn't really care about her U.K. fans.

But today, P!nk fired back via an open letter on her website ... saying, "I am a human being, therefore I am able to catch illness. It is beyond my control -- and I can't apologize for that."

"And you know what I've been performing through the last week? ROTA virus. Look it up. I challenge any one of you to do what I do while sick with that. You couldn't do it."

FYI -- Rotavirus is no joke. According to the Center for Disease Control, rotavirus causes "severe watery diarrhea, often with vomiting, fever, and abdominal pain."

P!nk actually concludes her letter by apologizing to her "loyal and understanding fans" ... saying, "I am so so sorry if I've hurt you in any way."