i don't know for sure, but yes, i suspect chunk loading.
the way the elastic ip is billed is a bit funny. first, there's a small fixed cost for everytime you "remap" the ip to a different ip, but that only kicks in after the first 100 remaps of a month, which we didn't exceed (a remap happens every time we boot the server).
second, we're billed half a cent for every hour the elastic ip *isn't* in use. with the reasoning being if it's in use, you're already paying for it.