This kind of bump can be a swollen lymph node,a sebaceous cyst or a boil. Lymphadenopathy (swollen lymph nodes) is usually caused by a viral or bacterial infection along with other numerous causes.Treatment is usually cold compresses,over the counter anti-inflammatory analgesics(pain killers) like acetaminophen or ibuprofen,antibiotics if cause of infection is suspected to be a bacteria and surgical drainage if there is any abscess formation.
Sebaceous cyst is a closed sac or cyst below the surface of the skin that has a lining that resembles the uppermost part (infundibulum) of a hair follicle and fills with a fatty white, semi-solid material called sebum produced by sebaceous glands of the epidermis.It is usually painless but can become painful if infected.Then antibiotics are needed.
Boils are caused by infection of the hair follicle be staph aureus.Treatment includes antibiotics and warm compresses.
I feel that an examination by a dermatologist is required to confirm a diagnosis as without examination,confirming a diagnosis is tough.Till then apply warm compresses on the lump and keep that area clean and wear cotton undergarments.In case of pain,you can take some anti-inflammatory analgesic(pain reliever) like ibuprofen.
Hope it helps.Take care and pls do keep me posted on how you are doing or if you have any additional doubts.
I do not know if mine are exactly like yours (like kindd explained, there are many kinds of "lumps"), but I have one on my right buttock and one on my right shoulder-blade. The one on my buttock is purpleish like you described, but the one on my shoulder is more of a yellow/tan. Both progressed about the same (starting like a large pimple in size, then growing) and hurt when pressed. Both are fixed and don't move when I manipulate them. That's one thing that distinguishes many different types of lumps--does yours move freely under the skin?
I do not yet know what mine are (I'm in the process of being evaluated for severe joint pain, etc. and posted a separate thread a few days ago about all that), but agree that a dermo should know what it is.
I will say, I got irritated with mine and messed with it too much in an attempt to "pop" it; I felt like maybe it was a very deep zit that I could just bust through the skin if I tried hard enough. D'oh! Don't do that.
If you hear more, keep me posted. I've been trying to find answers myself! :)