Nipples are sensitive, and they don't always react to any particular situation together. It could be your left nipple is more easily irritated, and that a recent change in laundry soap or shower gel has affected it; or perhaps some little imperfection in your bra. You may be feeling something hormonal; or, less likely, you may have an infection developing in the milk ducts around the left nipple. Please read our post on nipple issues for more thoughts on what might be causing this. Breast cancer is usually painless, so it's unlikely that would be the issue. However, if the pain doesn't go away within a few weeks - or certainly if it worsens - it would be best to see a doctor, to find out for sure what's causing it. Good luck - PJH