Purple marks on breasts, on the inner side only

hello,

I'm turning seventeen in a month, and have been in puberty for two years now. My breast development started a year ago, and I now have a 75B. The problem is, that I suddenly got purplish marks on my breasts, only on the inner side of them. The marks look like veins, but aren't. 

I feel suspicious of these marks, and kind of embarassed. 

Do you know what these might be?

Thanks in advance