breastfeeding and missed period

Just wondering is anyone who is ebf has had irregular periods. I'm ebf and got my period back after two months and have been pretty regular about every 30 days with a period. I'm ten days late now and took a pregnancy test in case and it was negative. My so and I have used condoms so I'm not thinking I'm pregnant esp with the negative test...anyone else have a similar thing happen or any thoughts? Thanks in advance!!