Concerned about my pregnancy. My uterus feels different during different times of the day!!!!?

This is my first pregnancy so maybe this is normal. I am 15 weeks pregnant and I have noticed that when i wake up in the morning my uterus is fairly soft. But then I go to work and eat lunch (by the way i work at a bank so i dont do anything strenuous at work) and I notice that my uterus is hard and very easy to feel. Is this normal??