When DS1 had bad diarrhoea, around the same age, the Dr said to just give him extra fluids - which you are doing. As long as he kept eating and drinking it would go away.

Sometimes you just have no idea where the bugs come from. DS1 had it and no one else did, they must just be extra sensitive. It's pretty awful for babies because they're so vulnerable, but they seem to get better just as fast as they got sick.

Of course, if you're really worried - visit the Dr and just make sure.