It’s not embarrassing at all. It’s a bit gross but so are lots of things in life we have to deal with, so yeah, it’s great you asked!
So, the fact you’re getting poo on his cock all of a sudden might indicate you’ve loosened up your anal sphincters a bit too much by doing a LOT of anal. However if you’re not noticing any problems beside sex then I wouldn’t think it’s a big issue. Maybe just ease up a bit on how much you’re doing it and see if it makes a difference.
But besides that, you need to explore a better washing out regime. Most find that a simple little anal bulb douche two or three times an hour or so before sex will rinse out anything near your butt and leave you good to go.
There’s other options too, but I won’t go into detail, here’s a good write up: https://thebiggayreview.com/anal-douche-guide/ – any of those will help, just by washing out any poo that’s near enough to cause problems. But note you don’t want to go too far with it as then it becomes a ‘colonic’ and washes out MORE stuff from your lower intestine, so doesn’t help at all.
So, I hope that helps. Some find the whole douching and enema experience REALLY hot btw, so see if it’s a turn on for you or just a practical aid to keeping anal sex as squeaky clean as anything shoved up a butthole can manage to be!