The free ships (Increased Ship Building) that Japan and United States receive during 4 rounds, must be placed in each round as they become available - after the US player has completed the US turn.
So, in your case you must place the US Cruiser unit and Submarine unit in sz 10.
Part of the Strategic Plan for both Japan and the United States should include a plan for the use of the free ships - and anticipate a possible scenario as the scenario that you experience in your recent game.
A good idea is to place the free ships on the side of the game table, so you can easily see when they become available.