3rd pregnancy, 4 year old says twins mama!

So im pregnant for the third and final time! And im so excited! I have a 4 1/2 year old son and a 2 1/2 year old daughter. We were talking to our son about mommy having a baby in her tummy, because i dont think he remembers when i was pregnant with his little sister. I said Mason do you want a boy baby or a girl baby (thinking he would most likely say a little brother) and he said well mama you are having not one baby but two and their twins! He then said mama, there already girl babies. I was shocked! Since then He brings it up evernight! There are no twins on either side and i dont even know if hes ever even know about twins. However this pregnancy has been SOO different. I usually never loose energy but do get a little sick (around 9 weeks), i have been exhausted for 2 weeks now and im only 6 weeks! And i know its my third but i already have a belly. This is crazy to me! Any thoughts or has this happened to any of you. Please let me know! thx!