Putrification is second stage... starts to liquefy.
Bloat stage: when microbes in intestinal tract start to increase in number because of all the nutrients being released from decomposing tissues. One of the byproducts (waste products) of bacterial metabolism is gas production. Ammonia, methane, etc.
Sometimes in death the body exits (colon, etc) get blocked and gasses can't escape and build up in abdomen, or wherever bacteria is present and exits are blocked. Eventually those gasses will find their way out.