I think the itchiness is caused by something you have been wearing and if not it is because you are going through puberty if you are a teenager. Another reason would be an infection of some kind and you will need to see a doctor as soon as possible to determine the real cause for this.
My nipples used to get irritated at a job I used to have. My job involved getting splahed with water all day and going briefly out into the cold repeatedly. After a little while of that, my nipples were chafing on my t-shirt and getting raw. The only solution I was able to come up with was wearing bandaids on them at work, at least for the winter. Is that what's happening to you maybe?