Character Ability Score Improvement Feat no long available

My character is a level 5 fighter and had the Shield Master feat Improvement on his character sheet which allowed a shield bash with 1 attack on turn. I clicked on a different feat to see what benefits that had and now I can no longer go back to the Shield Mastery feat. This was in the 4th level Ability Score improvement slot. Why does it not provide me with the option for Shield Master anymore? Is this a bug? I had no option to undue as well or not save my choice. 

Shield Master is from the Player's Handbook, so to use it in the character builder you need to either own that book or have access to it through content sharing. If you had access to it before and no longer do, it may be for any of these reasons:

• You left the campaign where the content was being shared
• Content sharing was turned off in the campaign where it was being shared
• The person who enabled content sharing canceled their subscription or let it lapse
• The only person in the campaign who owned that book left the campaign