@Pikachuka My DS did the same thing, I started a thread about it because I was mildly worried too.
We had a nice reassuring chat and someone posted a link to a guy who had a sapling germinate in his lung because he inhaled a seed (from a pine cone irrc) and all was well! (With us, not with tree guy, though they did get it out.)
I didn't poke about in poo and no ill effects at all. Nowadays he doesn't eat plums or much fruit, so I look back on the plum stone episode with nostalgia.