Help! My gf won't talk to me cause I keep on asking her to hi-5 me (she has a prosthetic arm).?

Ok I've been dating my girl for about five months and she is a great woman and she lost her arm as a baby. Anyway she has a prosthetic arm ( an old type one that basically just stays in the same "mid handshake" position). Well, as a goof I like to say "Hi Five!" and she'll get mad but this time she hasn't talked to me in four days. What do I do?