I'm a 26 year old male with a sore lump behind right nipple, sometimes i get it in both nipples and it goes away, has been happening since a teenager. any idea what can cause it?

What you described suggests enlargement of male breast, gynecomastia. Visit this site for information on this topic and talk to your doctor as the condition is treatable: https://my.clevelandclinic.org/health/diseases/16227-enlarged-male-breast-tissue-gynecomastia#:~:text=Gynecomastia%20is%20an%20enlargement%20or,of%20balance%20with%20testosterone%20levels.

Men also have milk ducts in their breasts, though usually they are dormant. This could be a clogged milk duct or perhaps a cyst. Do warm compresses help? You should mention this problem to your primary care provider and consider further evaluation, possibly a breast sonogram and possibly hormonal evaluation especially if there are any other reasons to assess hormonal balance.