Help! What does this mean?

I had bloods done yesterday, and the lab called today to say I have a dangerously low white cell count - 0.3. I have got to go to the day unit on Monday for more tests, and he said that I need to monitor my temperature all weekend, and if it goes above 38oC I have to go straight to the hospital for IV antiobiotics. I am really scared now. Is this that serious? What could it be. Any help or reurance would be great. Thanks.