Missed period?

I’ve been expecting my period for a little while and haven’t gotten in. I’ve always been on birth control but haven’t been for the last few months. I use a condom whenever I have sex. My sister and I are usually on the same cycle and she’s well into hers for reference. I also have been with her a lot so not expecting to still be on her cycle. I’ve also taken one test and it was negative. Trying not to stress but I literally do not remember when my last period was, after being on birth control so long and not having a period I forget to keep track :/