I have two babies but still get jealous when people are pregnant?

Hello!

I have a 4 year old and a 8 month old. We would love a few more babies. I had a c section with them both and my OB reccomends 18 months before trying. My hubs is totally on board with having more babies and I dont have any issues getting pregnant. I love being pregnant and right now I am enjoying both my kids. But I get so jealous and a bit sad when friends or fam annouce they are pregnant. I can truly get happy and excited for them but I also seeth inwardly with jealousy. I have no idea why im like this? Is anyone else like this despite having babies and the fact that you will have more? How can I not be like this?