Baby sleeping more at night, but no naps?

For the past 3 nights my son has been sleeping 13+ hours every night with no wake-ups. During the day, however, he has suddenly stopped napping. It makes me nervous because this is what he did right before his 4-month regression. I have the Wonder Weeks app, but it doesn’t give me any explanation as to why he’s doing this. He is fussy during the day because he’s tired, and he’s generally been acting weird the last couple of days. He gets super happy, laughing like crazy at everything, then he’ll get super somber and quiet. Anyone know what could be going on? Is this normal behavior or should I be concerned? We started solids recently and he’ll be 6 months in a couple of weeks.