Is it normal that my wife refuse to make love with me?

Our sex relation is decreasing since 2 years after 9 years of marriage which cause me a lot of anxiety and stress. I start chasing my wife when she goes to work, shopping, beach, meet her friend and even when she goes to her parents. I have a doubt that she is dating her boss or one of her colleague and try to talk to her about that, but always she gave the same answer, that I’m misapprehending a lot and that our sex relation is decreasing normally since we are getting old and due to work stress. Two weeks ago, We planned to go spend the week end on the mountains since the children can profit from the green nature and me & my wife we can benefit from some relaxation too. After finishing the arrangement of our things and while the children’s where playing in the yard and between the trees, we decide to go take our bath. Everything feels normal, we take the bath together and share some hugs and kisses under the hot water and when we finish, we went to the room to dry ourselves, I try to push her to lie down on the bed, but I felt some resistance and refused to let me take off her towel, she told me that she is tiered and prefer to keep it till midnight after the children’s get sleep. I put on my jeans and shirt and left her in the bed lied to relax. 15 mnts later I decide to return to check my mobile phone where I caught my wife still on the bed cover with her towel while her legs were spread wide and masturbating with an old vibrator that I offered to her 5 years ago and she never use it or let me use it for her. She always has an objection of using toys. I start enquiring, why she refused me and start masturbating, why she always refused in front of me the toys and now she is using it with pleasure, and why she bring it with her from the safe box to the mountains if she hates it. I still not have a convinced answer on that.