Sadly, this sounds much like my daughter at the same age. It's only really changed in the last month or two since she has become more adapt at ridding herself of wind.

Explosion in the nappy is exactly how I describe the poos too. She grunts and wiggles and sobs at times and then when it's all out she's as happy as larry. It's like they don't realise that they don't need to strain quite so much to get loose poos out IYKWIM?