Hi JustMe-
I think that the fatigue and lack of focus you describe is fairly common in the recently diagnosed.  I certainly suffered from the lack of focus!  The reasons are probably a combination of physiological and psychological.  Stress can definitely affect your abiltiy to conentrate - and I don't know of too many things more stressful than being told you have a brain tumor!  

Give yourself some time to process everything.  And, if you can, try to lighten your load at work a little.  As a business owner, I found that last bit of advice a little hard to take myself, but it is worth it if you can do it.  
The decision to tell the people you work with about this is highly personal; in my case, I finally decided to tell my inner-circle of colleagues because I felt that they needed to know why I was so forgetful and "not present" sometimes.  They were all amazingly supportive, which was a big help to me, both before and after treatment.
Hang in there!
Debbi, enjoying fall in NJ