Is it ok to travel without a companion if I am pregnant?

Yes. The answer to this question is really "it depends..." there are so many factors involved at different stages in pregnancy with the presence/absence of other complications, as well as travel mode and destination that you need to discuss your trip plans with your doctor.
Travel. It's ok to travel if you are in the second trimester and your pregnancy is uncomplicated and your travel doesn't involve any prolonged sitting or other unusual stress. Travel in the third trimester should be limited and not recommended without a companion. Travel in the first trimester should follow the same rule as in the second one.