A feeling of jealousy?

I just started dating my partner 20 days ago and I already have feelings of jealousy. I wasn't told he was Poly until just yesterday and I'm perfectly fine with that because I'm a little Poly myself, but it still gives me feelings of jealousy. I'm the kind of person who'll date two people and be fine, but when my partner dates two people, I get jealous. His other partner is hot, don't get me wrong, but I would have appreciated it if he had told be before, especially because he was dating someone before we started dating.