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.