What Causes Pus Filled Bumps On The Back?

hi, i have always had a spotty back ever since a teeneger (i am now 33). for about a year or so i have occasionally found lumps beneath the skin (not prominent or obvious spots) which when squeezed excrete a fairly large amout of a white, extremely smelly puss. the puss is not accompanied by any blood and is fairly dry. is this unusual?