I got skin growth under left breast during my pregnancy and its growing.its now half cm in 2 yrs.looks uneven.red,bubbly.twice in 1 yr itches.harmful?

Not likely: when there is friction between to areas of skin you can develop a "skin tag". they are common at the neck, arm pit, groin, and under the breast (particularly with heavier breasts) They can be easily removed.

See a doctor: Based on your description, a fungal infection commonly known as "tinea" would be most likely, particularly if you have pendulous breasts, as the warm environment underneath the breast makes it more likely for a fungal infection to grow. However, if it has been there that long and is growing, there is strong consideration for immediate biopsy which would require in-person consultation.