Has this happened to anyone else???

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I'm currently nursing my 21 month old. I got my period back almost immediately after having her. But it randomly stopped after having it all this time. July 26th, 2017 was my last period. My doctor did a pregnancy blood test to see if I was pregnant (also TTC) and it was negative. She said if I don't get my period back by the end of November then she'd have me come in to do some more testing. Has this happened to anyone else or heard of this before?