Weird question EBF and Periods

Jacqueline • Proud Momma of a 10 year old Girl and a baby Boy born 09/13/18 ❤💙

I have EBF since my son was born. My first period I got was in December. Then I got it again in January. I am now 5 days late. I have taken 3 pregnancy tests all negative. Can breastfeeding cause you to sometimes get your period and sometimes not? I thought once you got it back then it's back for good.