Anika - I have been back at work for about a month now. My first week was hard, but it got better after that. (I went back on a Tuesday, but it sounds like you are going Monday. If I had it to do again, I'd make my first day back a Wednesday!) One thing that helped me was to be very honest with my boss (and I am lucky to have a great boss) about how hard things were, how sick I got, and how I felt unsure about being back at work. My office has let me set my own pace, and I'm gradually having an easier time focusing on my work. For the first two weeks, I really got almost nothing done.
I also found it helped to choose a trusted colleague or two to ask to have lunch in case no one asks you - many folks at my office lunch every day at their desks. This helped me to get back to more normal contact with my closer colleagues, amd since I am closer to them, I was able to share a bit about my experience and it helped things seem less awkward.
One other note - get yourself prepared for the oddball reactions. My counselor suggested memorizing a line or two response for when people say well-meaning but distressing things. I think I used "It's been hard, but I'm getting better" and a simple "thanks for your concern"
Good luck. Let us know how you are managing.
Mom to Amelia, b/d 03/19/04 @ 22 weeks due to sever PE