I've had something kind of similar... Except my jealousy was over my interest, who I am NOT dating. I think in this case, it's because I don't know if he might be mono, which is likely, and if he is, meeting someone else means I'll never "have" him.
On the other hand, I feel compersion for him in other areas, such as when he moved for work reasons (so I totally relate to what TP is talking about).
I don't experience jealousy in my existing relationships, but I think it's because I have absolutely no insecurities about them.
So I would say your instinct is right, that's probably exactly what it is.
Me: 30F, straight
Seamus: My husband, 32M, straight
Dragon: My boyfriend, 29M, pansexual, married to Fox
Fox: My boyfriend, 29M, homoflexible, married to Dragon