13 month old slapping me

My youngest baby is 13.5 months old. She is the sweetest little thing and so snuggly. I know she isn't doing it to be cruel but for some reason she has decided to test me by slapping me in the face repeatedly. She is relentless - if I hold down one hand she will slap with the other. She doesn't laugh while she does it, she looks as if she's concentrating. I have been holding her hands and telling her firmly "No!" and if she keeps doing it I put her down and ignore her for a minute. She cries and pulls at me and then when I pick her back up she starts doing it again. We do not hit in our house at all so she has not seen this from someone else. Maybe she likes the reaction she gets from me, I don't know. How do I stop her from doing this because it is starting to upset me.