My nipples are turning hard. Why would that be?

Oh ,my nipples are so hard

My nipples are the reason why you will get hard today

My nipples are always hard

my nipples are sooo hard…they need to be sucked on right now

My nipples are already hard

My nipples are getting hard thinking about you under me

My nipple are getting hard!

My nipples are always hard and ready to be sucked

my nipples are always hard

My nipples are permanently hard since I got my new piercings

My nipples are always hard.. is that a problem?