Tell her you suffer from a condition called OCD. Explain to her what it is, try to help her understand as much as she can, so she doesn't get the wrong end of the stick, and so maybe she could help you more with it. Try to remain calm throughout, as being calm does help. Remember; she's your godmother, and she'll love you either way, so don't be ashamed of sharing this with her, who knows? It may open a door for you...