Having a lump doesn't automatically mean it's breast cancer. Nor does it being painful make it breast cancer. Cysts (fluid filled sacs), benign tumors (non cancerous), among other things can cause lumps that may or may not be painful. YES, you do need to get it looked at by a doctor, because cancer can't be ruled out until it is, but try not to freak until you get results.
You doctor will want to know when it first appeared, if it changes with your cycle, and how it feels. Also tell your doctor about any pulling, itching, burning sensations, nipple discharge, or changes in the skin. Your doctor will probably order an ultrasound. This is normal, no worries if they do. Dr's usually use an ultrasound over a mammogram because it will give a clearer image on younger woman. An u/s is also usually better than a mammogram at helping them figure out what kind of tissue the lump is. (At least that is what I was told when I was 19 and had it done.) If they think it's even a tiny bit suspicious they might order a biopsy. But even these can come back normal, so do your best not to stress for now and make sure you see a doctor.