My dog randomly just growled at me?

Today My dog was eating a bone, and my little brother went up to her and she growled at him, then I tried it and she growled at me too. She is 3 years old and has never growled at any person before, when she was a puppy we would bug her when she would eat to prevent this sort of thing. I am just worried that something may be wrong with her. She would growl at me when I brought my hand close, but when I would touch her mouth (I wanted to see if she would bite me) she wouldn't, instead she licked me. When I took the bone away, she just went up to me and started licking me (so she wasn't angry with me at that moment). I may be over reacting but I think it is odd that when I approach she growls but when I get obnoxiously close she licks me. I was just in costa rica for 9 days and got home yesterday and she stayed with my best friend at my house, could this be something that caused it? Why could she be growling?