lve had this small brown lumb on my back for many years but never understood what it is. it's not really itchy or painful have been swollen sometime when i have layed on a spikymatress or just on the floor but it usually gets better within minutes, anyone know what is it?

Its a skin tag which are not cancerous and no need to worry about it. they can be a problem if they catch or rub on your clothing. If it really bothers you and it has a narrow stalk you could ask someone to tie and knot a piece of cotton thread very very tightly around the stalk and just leave it. Dental floss is ideal .
This will cut off the blood supply to the tag and it will eventually drop off.