My 5 year old has been acting out

I'm due May 14 with my second baby and for the past few weeks my first born Has been super whiny and bratty. He is usually petty well mannered but I have no idea what has gotten in to him. He is been giving me a nasty attitude and super impatient. I've also had the hardest time having him eat. I have asked him what's wrong but he just says I don't know. He seems distracted. Has anyone ever experienced anything like this.