Most Helpful Guy
Part of the problem here, is that the anus and bowel are not designed for sex. What would not be a problem with the vagina, can be a problem with anal. that's because the bowel wall is thinner, and more prone to injury and tearing than the vagina. Also, being as there is poop in bowels, various things can bring it out sooner than normal, and in different consistency. Stimulation is one way of getting the bowel to move, and sticking penis in is stimulation. Repeated anal penetration can lead to sphincter muscles weakening, leading to urgent needs to defecate. Some diseases are more easily transmitted via anal sex, viruses that lead to rectal cancer, and HIV is much easier to contract with anal sex.
Most Helpful Girl
It's possible he didn't wash his hands and some bacteria or even chemical on his fingers made you react and your guts flushed out in defense (like getting food poisoning). Or its unrelated to his finger.
Just be clean, wash all hands and anything else thats going into and has come out of anyone anywhere (not just butts!).
If this happens again you might have an issue, but I'd just stay closed back there til things get better, then maybe explore yourself in the shower, or with a toy first, figure out if its all ok, then give him the go ahead.
Other point - anal is for grownups. If anyone gets grossed out by accidents or anything getting slightly dirty (shower first though) and makes their partner feel bad for it, stop doing anal, you're not ready.