My interpretation is pretty simple, but at the end, Anastasia is not consented anymore, and the scene becomes more of a rape than anything else. Katherine O'Clare, of Crushable, wrote :
Christian Grey is an abuser. He is emotionally unavailable, emotionally abusive, and sexually exploitative. His complete disregard for his partner’s comfort, experience, and emotional well being are contemptible: he gets his pleasure at the expense of his partner ...
All sexual relationships, whether vanilla or kinky, require consent. There is no such thing as having sex that is not based in consent. The only thing that happens without it is rape.
And that is what happens in the last scene. This movie really isn't about BDSM, which require trust and respect, and not an informal business contract. These tears clearly show she does not want to be there anymore, and the fact she leaves shows that beyond her love for him, she knows she has been a victim.