3 kids fell pregnant 1st month all 3 and now 4th is taking forever.

Stacy
So my first 3 I fell pregnant super fast, like first month fast, so just assumed it would happen like that again. 
5 months later I'm still here and another period has just come. :-(
Anyone else had this and fertility changed after so many kids? 
What could be wrong that wasn't before? 
Periods are regular and normal flow.