I think it's just getting older. Their urine gets more concentrated and the older they get the more their poop is 'adult-like' (even if they're still breastfed). I found the same thing with all my kids and only had bamboo when the twins were 18 months.
I'm totally with you on the frequency of baths, mine are still pretty much on that schedule unless they're particularly dirty from playing in the sand or something. I've found that using some kind of wipes solution helps a lot with this. You wouldn't have to use it everytime, I found that first thing in the morning and after poops was fine for us. I've used everything from pre-made solution (you can buy some that's concentrated and you dilute as needed, some even have foaming dispensers you can put directly on baby and wipe off) to stuff I've made myself using a few drops of shampoo, some kind of oil (I had some lightly scented massage oil that wasn't getting used so I used that) and maybe a few drops of tea tree oil to help keep it fresh. Lots of other recipes out there too...