Why does this bother me???

🌸Karaline🐝 • Breathe deeply, tomorrow isn`t yesterday
Last weekend my fiancé and I went to a wedding of two of his friends (who I've never met). One of the guests was my SO's ex and she literally followed us around at the wedding the whole time, this was my first time meeting her. At the beginning of our relationship she had asked him if he'd ever give her another shot, so she's already not on my good side. What is still bothering me though is her claiming that she is pregnant and "due" in April, which means she's 8months and huge right? No, this lady could possible be three months pregnant but the only thing showing is what I think is left over baby weight from her baby born in 2013 (ok, I FB stalked her after the wedding).
Why am I so bothered that she's lying about being pregnant???