What Causes Blood Filled Blister On The Anus?

I have a pimple very reddish on my anus (well at least I thought its a pimple) tried to pop two days ago with no success but today it was bleeding and now it bleeds not all the time but when it does it bleeds non stop and alot since about 3 hours ago. This is the worrying me that what if its a vein? Its not as painful as when I noticed it. Actually its not pain any longer but the bleeding is worrying me now.