Large lump starting at anus and goes all the way up to the start of scrotum. no itchiness or discharge. painful?

It is painful?: If it is painful, it is very likely to be an abscess - a collection of pus under the skin. Start with soaking and ice, but if this gets bigger, more painful, red, or starts to weep, you will need to see a physician. If this is an abscess, it will need to be opened. Also could be thrombosed hemorrhoid. That will get better with time, soaking, and ice.