WTH is wrong with me

I’ve been with my husband for over 4 years and we were on and off for 6 years. All the sudden the only other guy I saw spending the rest of my life with added me on Facebook again. We haven spoken since I told him I was engaged. (He was pretty upset that I was getting married) it’s now been 4 years and 2 kids and all the sudden I’m having jealousy over my ex... he posted a picture with him holding a baby and I can’t tell if it’s his or not... not that it matters because I love my husband very much. Someone tell me I’m not loosing my mind and it’s normal to think about him every now and then...