In need of advice...

So I am 26 almost 27 and unfortunately when I was 25 I went to see why my Bf & I were having difficulty getting pregnant. During my appointment I was told I had have a pap and about a week later my doc called me and requested I go in Asap. When I went in I discover I had cervical cancer (fortunately in the beginning stages) and needed to have part of my cervix removed. Because of this I have a slim chance of ever getting pregnant. I came to the conclusion that we could not have children so I thought I had made peace with that part of my life. About 2 weeks ago my now fiance's little cousin (21) told us she is pregnant and for some reason I was filled with so much jealousy, anger and sadness and I do not understand. I have a sister and a sister in law that have had children recently and I felt nothing but pure joy and happiness for them. I don't understand why I feel this way towards the 21 y/o and not my sister in law and sister. Am I wrong?