Diarrhea. "Backdoor" refers to the now-outdated practice of using an outhouse. Something I ate has given me a serious case of the backdoor trots, ugh.
See also: backdoor
See backdoor trots
n. a case of diarrhea. (From the time when people had to go out the back door to the outhouse.) I can’t go out tonight. I got a case of the backdoor trots.