Firstly, that sounds very normal to me. I felt for a long time, that being poly meant having no bad feelings about things. It was through this forum that I realised... emotions are what they are... and we don't have to be robots.
For me, at least, it seems that emotions are particularly strong when we face any kind of new situation.
Some people also feel emotion more strongly than others.. i.e. crying.. whilst some people only have minor twinges.
Now, why is it that you can't experience intimacy with her? Are you in a primary relationship with B and are only supposed to see P sexually?
How about this thing where you can't sleep with other men? Are you happy with that arrangement?
I think that emotions are complex things. It seems that you are feeling a little bit of insecurity and a little bit of envy.
The problem with envy is that is can breed resentment. There's absolutely nothing wrong with feeling it... but you have to try to address why you're feeling it. When nobody but yourself is standing in your way, then it's just envy. If someone else is preventing you from having something, envy can turn into resentment towards that person.
I do also feel that emotions can hit us at unexpected moments. A few months ago, my girlfriend and I went to an event and ended up playing around with a girl together. It was only kissing and touching, no sex. It was fantastic at the time. Hours later, when I went to bed on my own, I had these strange pangs of sadness. It really disappointed me that I was having them. But I worked them out in my head and basically, I was just missing a connection with my GF, because we'd hardly paid each other any attention all night. I felt a little guilty.
Usually, we can pick apart emotions and this helps us to deal with them. Other times, we know what they are, and just have to ride them out.
Either way... what you are describing sounds perfectly normal... and I'd love to hear more information if you would like to share.