In your case, you seem to lose 4 slots when this happens, and going by your cell (microwave, bride/groom, clovers and cake) you add 4 slots, so it would appear my suspicion might be correct. Now to figure out why the heck it happens
yeah definitely related to the amens. I take out the microwave, then reload my bag and i only lose 3 items at a time. it even happens when i'm not in the yard and just put items into my bag from my items page. I refresh the page and I have 3 open slots again and my item count for small waters increased by 3.