How can I stop my girlfriend from biting me?

We've been dating for 3 months and she has always been biting me. She constantly gnaws on my arm like it's some chew toy. I ignored it at first as it was funny and ticklish but now it's starting to get annoying. She sometimes even bites me when I'm sleeping (like 2 or 3 times). They dont hurt but it's kinda annoying me. How do I stop her?
How can I stop my girlfriend from biting me?
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