Yes, it happens sometimes with raw milk. If it is what we call here "late bloating" then it is a contamination with buteric acid bacteria. Silage can be the reason, but it is an external contamination caused by bad hygine during milking. Silage makes the manure thinner and containes a higher amount of bacteria, so the risc is higher. Here it is allowed to add a certain ammount of saltpeter to the milk to prevent this late bloating. Danisco Holdbac is also ment to prevent this.