Relatively common in older males. The soft faeces that guinea pigs re-ingest as part of their digestive process accumulate in the peri-anal sac creating a 'lump' in the guinea pig's bottom. This is partly due to a weakening of the muscles but is worsened by a low fibre diet.
Vitamin E, found in wheatgerm, is helpful in strengthening muscles long term but the guinea pig will probably still need your help to remove the lump in the short to medium term. Lay the guinea on a towel, on your lap or the crook of your arm depending how confident you are with him and apply a few drops of edible vegetable oil or aloe vera gel as lubrication to loosen the lump. If done regularly the lump will pop out with a little gentle pressure but if it has built up unnoticed you may need to gently lever it out with a cotton bud.
If the guinea pig wants to eat the lump then let him as it is a rich source of B Vitamins and predigested protein. Boars that are regularly impacted should be giving toasted wheatgerm as a source of B vitamins and vitamin E plus a good probiotic. They may eat the soft droppings of a cage mate so if you see them begging at a companion's rear end you'll know why!