Judy,

The same thing happened to me with my son.  He was also 9 months when my
period resumed.  It was my wakeup call as my period did not return with
his older sister until she  was 20 months.  We were in the middle of a
move at the time.  I sat down resumed nursing on his schedule.  No period
again until 18 months.  By then, he was only nursing 3 times a day.

Aloha,
Gloria Thai LLLLeader Hawai'i