Nipples hurt when laying down?

Okay so I’ve noticed that when I lay on my stomach my nipples get irritated? I wear a 100% cotton t-shirt to bed so I was thinking maybe it’s that? I don’t get it any other time than when I’m laying in bed. I have B cups so I don’t think it’s the size and they don’t look red or irritated but it’s like they burn? Doesn’t happen any other time I’m laying on my stomach either