The very initial stage of your pregnancy, weeks 1-4, includes at least one week in which you are not actually pregnancy but in the pre-ovulating stage of your monthly cycle.
You are ovulating, and your egg could get fertilized if you have unprotected sex this week.
This week the cells in your fertilized egg are dividing to create many more cells, as the egg travels down to implant itself in the lining of your uterus. You are officially pregnant, although you still may not feel like it yet.
The egg is continuing to implant itself in your uterine lining, as structures are beginning to develop inside. You may be spotting this week, and as the pregnancy hormone is released into your body, you may feel nauseous, achy or tired.