Hi, i think i have a cyst just below my left breast. it feels like a lump but im not sure if its water filled or rubbery. its just a little hard. do i need a mammogram?

You need a breast US: Based on your age, I would recommend getting a breast ultrasound (US) to start. If it's a cyst, it will be clearly shown on breast US. If it's a mass it will also be shown. After US, further imaging or treatment can be performed based on what it looks like. In someone young like you, it is most likely a cyst or fibroadenoma. Both things which are benign. Keep feeling it to see if it changes.