You need to come clean, now--your lie will probably hurt your relationship a little bit now, but it'll definiitely hurt it a lot in the long run if you don't take care of it quickly. You need to explain why you lied to your girlfriend, and you need to clearly and carefully let her know the truth, and hope that she's accepting. (If she needs reference material, send her to this section of this site where she can ask questions of others with this fetish.) TCC pretty much has it right--this is the type of thing you want to make known early in a relationship, and the type of thing that you want to handle carefully. If you keep this secret, the truth can and very well may come out eventually and then you'll have far more explaining to do, and a far larger hole to repair in your relationship, if it can be repaired at all.
My advice would be to take your girlfriend out for a nice dinner, and then afterwards, begin by apologizing to your girlfriend for lying and actually tell her the truth. More than likely, she's going to be angry with you--not about the diapers, but about the deception--you need to explain that your intentions were good, but your actions were poor, and to take it from there. The more sincere you are, the better off you'll be--if this is enough to drive your girlfriend away, then she wasn't right for you in the first place and you should seek out another partner. If she's tolerant of your fetish, the lie will still hurt you for a little while, but that will pass and you'll be able to move on with your relationship once it does.