Jealous

I don't understand this! Recently I've been getting jealous of women who are pregnant/have kids,I've always looked at these women and been happy for them. This all started a few weeks ago when I found out a family friend is pregnant and I was of course happy for her but I also was jealous. Then a few days later my SIL told us she was pregnant and just like before I was super excited but also jealous. I'm still young(19) and I know I'm not ready for a baby yet but there is a part of me that wants to be a mother so bad! Has anyone ever experienced this? If so how did you get though it?