This has happened to me too. I checked steemd.com and found my remaining bandwidth to be at -16.08% :(
This article is pretty helpful: https://steemit.com/steemit/@rycharde/why-are-so-many-users-hitting-their-bandwidth-limit-solved-it-what-you-can-do
Update: I just checked again and my remaining bandwidth was up to 9%... which was enough to post. That took about 10 minutes. So the solution seems to be patience.
Yeah, it seems the more Steem Power you have the more bandwidth you get.