Praise be to Allah.
If a person delays zakaat al-fitr although he remembers it, then he is a sinner and he has to repent to Allaah and make it up, because it is an act of worship that remains due even when the time for it has passed, like prayer. But as this questioner states that she forgot to pay it on time, then there is no sin on her, but she still has to make it up. With regard to there being no sin on her, the general meaning of the evidence indicates that there is no sin on one who forgets, but she is still obliged to make it up for the reasons given above.
And Allaah is the Source of strength.