after child birth the time of occurrence of periods defer from person to person, it mainly depends on you are lactating or not,
if you are giving complete breast feeding the periods may not come up to 6 months and also it will give protection against pregnancy
if you are not lactating there is chance for starting of menstruation
early and also possibility of pregnancy,
so if you think that you are having possibility of pregnancy better to go for blood test
to detect pregnancy once.
for more details on this you can see my article,