Period then no period??

Hey ladies! So I had my son may 28th. I got my period the first week of July. Had one is August as well as spotting in the middle of the cycle. For September I didn’t get a period. I tested for pregnancy 3 times (after it was expected to come September 17th) and still negative test. I’m an exclusively breastfeeding mama. He nurses and receive breast milk in a bottle when I’m away. I pump at work. I don’t understand why I haven’t had my period yet. I keep having a lot of CM. Can someone help?? I called my obgyn she said we just have to wait and see to either get a period or a positive pregnancy test. I’m a FTM.