It's important to remember that a betting round is not over until every player has either matched the highest amount wagered or folded. When this is complete all the wagered chips are added to the total pot and a new round of betting begins. As mentioned previously, the player left of the button acts first again.
Please note that placing a forced blind counts as an action so during the pre-flop (first) betting round it is the player in position three from the button who acts first since players one and two are forced to place blinds.