A cyst in the breast is usually benign and you don't have to worry much about it, but it's not right to completely ignore it. I would suggest a complete evaluation at a clinic by a general surgeon. And you will have to get an ultrasound scan of the breast done to diagnose if it as an actual cyst (fluid filled cavity) or a fibroadenoma which is also benign and very common among young females.
Next Steps
consult a general surgeon
ultrasound scan of breast
fnac, if requested by the surgeon
Health Tips
you have to do a self examination of the breast in front of a mirror regularly to look for any new onset changes like lumps or ulcers or deformity.
it helps in early identification of breast abnormalities including breast cancer