period late but negative HPT...

Vann • Wifey 💑 and mom to Joey & Adrian 👦🏼👶🏼
I had my son 6/14/16. Vaginal birth, 2nd degree tear. He's been bottle fed since 1 week old. I bled on and off for about 6-7 weeks and then had what I assumed to be my period from 8/3-8/9. It was fairly heavy, but nothing awful. 
This month my period time has rolled around and I have been crampy and very PMS-y (grumpy lol) but it's been 3 days and no period. Took a HPT yesterday and it was negative, but I'm still paranoid.
Never had this issue with my first son. I was pumping/ supplementing and my period came back around 3-4  months and was normal. Should I be concerned? I'm still waiting to be fitted for my IUD :(