My nipples are itchy all the time, and my left nipple is painful.
Winter is a time of year when your breasts, particularly your nipples, can easily become dry and chafed. Have you been using a new shower gel or laundry detergent? Wearing a new bra, perhaps? Doing a lot of swimming? If you can't think of anything that's causing this, it could simply be the season. Please read our post on nipple issues; that'll help you decide whether/when to see a doctor about this. You might also find our post on nipple pain and soreness helpful.
Since it's happening in both breasts, it would be very unlikely this itchiness is a breast cancer symptom. Still, if it lasts longer than a month; or if it gets worse, it would be good to make an appointment to see someone.