missing period not pregnant

Li
I'm curios if anyone on here has missed their period but wasn't pregnant,
I finally got my period last month after 3 months of delivering my little guy and this month I'm a week late, I've taken a few pregnancy tests and they've all been negative.
What could it be? I've never been late on my period, this was my 3rd baby I didn't breastfeed before, so I'm thinking that could be the reason why my period is late? 
Any ideas ladies? 

Glow Resources

Let’s Glow

Glow is here for you on your path to pregnancy

Glow helps you navigate your fertility journey with smart tools, personalized insights, and guidance from medical experts who understand what matters most.

25+ million

Users

4.8 stars

200k+ app ratings

20+

Medical advisors