I think also sometimes you go thru a small vein a little way down - may not aspirate, or bleed to much because of where its located, but will cause a bruise and sometimes a touch of subsurface swelling. Awhile back I thought I was getting an abcess and asked my sister who is a doc and she explained the above to me. I tell her I just do very light "anti-aging" test - shes not to crazy about me doing even that, but shes always known I'm nuts anyway!