You can thank the baby for that one! You're so close to your due date, and baby is crushed there so rib bashing is completely normal. Congrats on the baby :)
I'm 39 weeks 1 day & my ribs feel bruised.
Are you pregnant? If so, how far along are you? Later in pregnancy (if you're that far along) rib pain can be common, and is most likely caused by the position (kicking) of the baby.
Constipation, bruised ribs (from falling or something) can also cause this.