Left breast started hurting, found dime sized knot near armpit, was told breast cancer doesn't hurt, is this true? what else could it be?

See a Doctor: Any new lump in the breast should be a cause for concern, best handled by seeing your gyne or primary care doctor. While most cancers are not painful, it is not a reliable finding. Hopefully, your doctor will be able to alleviate your concern or refer you to a breast center/surgeon for definitive evaluation. I hope its a false alarm--good luck.

Likely inflammation: It is true that most breast cancers are painless in the primary site. There can be exceptions. More likely a newly discovered "lump" that is painful is a lymph node that is expanded in response to an infection. However, to be sure, you should see your physician promptly.

Abscess: Most, but not all breast cancers "don't hurt." but any new change in your breast should be evaluated by your primary doctor or gynecologist. A new change in the breast that hurts could be an abscess or an infected cyst and with luck that's all but don't let new findings like this go by without evaluation.