Have you asked her? She's really the only one who can answer this question.
How long have you been together? Have you made it so she's comfortable expressing her sexuality? Is your culture one where women are free to do that?
Also, women in even more liberal cultures, like the US, get really mixed messages about sexuality. Their men like them to do things they see in porn, their churches or parents have raised them up thinking that "good girls" don't do certain things, and some men get weird if women masturbate, and others want their women to do it.
Just have a honest, but really kind, conversation with her and ask about these things. Don't be confrontational, or accuse her of anything. Tell her you love her, and love her body, and love being with her in that way. Tell her you want her to feel safe with you.
Good luck!