Hi Nautilus and welcome
Firstly there really aren't enough details here to give the best advice but I'll take a stab.
It sounds to me as if your body is doing what you mind cannot. For most of us dealing with poly seems to be a very emotional and mental thing. How much emotional and mental processing have you done? If the answer is not much, then there is your answer. You need to.
Respect what your body it is telling you that you need to do: open up your head and your heart.
It could take years before a person is ready to be confronted with their partner having sex with someone else in front of them. You could very well be in shock.
Your feelings are valid, don't be afraid to look at them, even if they are poly negative. If you don't feel safe sharing your feelings with your partner, try them out here first.