It has been called "Morning Sickness" for hundreds of years, most women do experience it first thing in the morning, so that is why it got to be known as that, however, it can occur at any time of day or night.
Many women who are pregnant have morning sickness, but there are some women who get sick at different times of the day and night. I had a girlfriend who got sick right on the dot of 6 PM when she was sitting down for dinner. Her husband was so well aware of it that he would not start eating until she came back from the washroom. Poor girl. Some women get sick at different times of the day, or even when they have gone to bed.
If it is really bothering you, please see your doctor.
Unsalted soda crackers are excellent for settling your tummy as well.
See the related questions below for more information about treating the morning sickness.