Sometimes after i masterbate, my fingers are lightly stained with blood but i feel no pain what so ever.what could cause this? p.s.irregular periods

Irregular periods: If your periods are irregular then the bleeding may be occurring from either that concern or there is trauma to an area that you are masturbating.

Hard to say : Without further identifying if this blood from an external source or internal source. Have you considered using a mirror to identify if there are any external lesions on the skin/membranes of the vagina?

Are you sore?: Either it is menstrual blood or you need to use a lube for self-loving. KY jelly is very popular.