I am sorry to hear that he isnt feeling well, is his belly usually sensetive if you give him a little treat from your plate?? I would lean towards the sllipper too. And if he seems to not be getting any better, I would take him to the vet. You cant be sure everything he ate (slipper) is out. My Bullmastiff Hercules did eat a nipple from a baby bottle when he was little, he started off having very loose stool, and was throwing up, then it went over to, not much coming out, and when he puked it was only stomach acid. We took him to the vet, and they x-rayed and found the nipple, he was on iv-fluids for about 36 hours while they observed him, then he passed the nipple himself.
Needless to say, my daughter was taken off the bottle real quick after that lol....
But sometimes our big guys end up eating things that they can not pass themselves, so rather than wait too long, I would take him to the vet, if you see it is not getting any better.
Please let us know how he is doing, and good luck and give him slobbers from me
Marit