Why would I need to have an u/s done on my spleen? PLEASE answer.?

Hi. Well I've been through a battery of blood tests in the last two months, and a bone marrow biopsy. Today a nurse called me and said I need to have more blood drawn (yay me...) and I have to go in for an u/s of my spleen tomorrow morning? Why would they need to do this, and what can it tell them? Thanks in advance!