Late period and on bc

Hello. I got my period April 13th. I was supposed to get my next period May 15th. But I didn’t get it. I started birth control May 20th. But between the period I had in April and the missed period I hadn’t had any sex. So I’m wondering if you would have any idea of why I missed my period even before taking the bc? My periods are usually off a few days give or take but never more than 3-5 days. So I’m wondering if it was already late and then taking the bc made me miss it all together? I also took a pregnancy test to make sure I wasn’t tripping and it was my period in April and not spotting but it was negative 100 percent lol