So I really don't fully know where to start with this, but I need some help figuring something out...

I'm in a poly relationship, and well up till now it has gone by without a hitch. I've been seeing my 2 main partners for a little over a year and they each have there own other respective partners. Now... my issue is with my latest partner, she and i have been seeing each other for a little less than two months, and with all my partners we're always open and discussing how things are going... no issues. But for some reason... when she and I where talking the other day, and she was mentioning how she was getting along really well with another person, that they might start dating soon... i started getting these really intense pangs of jealousy, that I don't even get when the two I've been dating longest talk about other partners... So I'm just wondering... has anyone else experienced something like this? How did you cope with it? Is this just a one off? or am I somehow really possessive of my new partner and why? I'm so confused right now...