I'm a month pregnant so why havent I had morning sickness?

Morning sickness usually starts at around week 6 but could happen later.  It usually won't happen too much sooner because the main cause of morning sickness is the body's reaction to pregnancy hormones.  Until about week 5 to 6, the pregnancy hormones really aren't strong enough yet to be causing very many symptoms.

And - yes - some women never get morning sickness.  I never had any nausea issues with my first pregnancy.  With my second pregnancy, I had maybe about a week of nausea issues and it was at about week 8.

You don't have to get all the pregnancy symptoms to be pregnant.  You don't even really need any of the pregnancy symptoms to be pregnant (other than missing your periods and having hormone levels to indicate pregnancy).  Most early symptoms are caused by how your body reacts to pregnancy hormones.  Most later symptoms are caused by the size of the baby and how the baby is positioned.  After that - it is all about how your body handles those changes.

Everyones pregnancy is different some people never get morning sickness at all