You may actually be seeing TNFa! Its ~25-26 KDa initially, gets processed to the most active ~17 kDa form. It also has trimeric 51 kDa structures (which dissociate easily), and has a soluble receptor at ~55 kDa.
What Ab are you using?
And incidentally, is there a particular reason you are not using an ELISA - the sandwich ELISAs are pretty specific nd sensitive, and you can use ELAST to further imrove sensitivity, though I do not think there is any biological significance at those low amounts (essentially baseline for a given cell/tissue type, since many cells can hold preformed TNFa).