Is this normal??

Heile • 22• New mother to a little boy❤️• happily taken💍

I gave birth to my first child back in April 2018, and my period started back up after about two months. Came every month on time for about 5 months. Then I didn’t have a period at all in November. I was nervous that I was pregnant so I took 4 test. All negative. Then the first week of December I got a period, so I chalked it up to stress from my job/life at the time. Now I was supposed to start Jan 4th, but it still hasn’t came. I’m guessing I’m going to miss it this month too and I’ve taken 1 pregnancy test and it said negative. I’m not on any form of birth control. Is it normal for you to completely miss a period (now twice) after it starting back up after giving birth???