It is really common after a while for partners in a relationship to become more used to each other and for the sexual intimacy to decrease.
Relationships go through stages and initially each person is learning about the other, trying to impress them etc. With time, this stage wears off which, does not mean that there is less love in the relationship. You may be right- she may be bored or the other possibility is that she may feel secure in the relationship and therefore not find the reason to stay because she trusts you and knows you'd be there for her.
If you are planning on talking to her perhaps you'd want to plan a nice dinner first and then after everyone had had their meal and is all relaxed say that you want to talk to her over coffee or a glass of wine. Begging the conversation by sharing your observation and ask her how she feels. Make sure she does not feel blamed for anything. Also, it helps to make her feel special perhaps get her some flowers and a nice lingerie. Try to get her side of the story and if she feels the same as you or if she feels things are going well as they used to.