I'm 24 and have been told I was adopted but I think my parents lied to me?

I'm 24 and this evening I found out I was adopted, which kinda makes sense seeing as she always said she went through menopause and her periods had stopped when she was 46, I was born three months before her 48th birthday. I'm confused at why they lied to me and why their lies didn't make sense. I'm also confused because me and my middle brother look the same -you can tell we are sibling. Same nose, same jaw, same facial structure, same eyes- but there is no record of me being born. Of my two brothers there are pictures of them as babies in the hospital, videos of their births but nothing of me until I was two. No pictures of me as a baby, nothing of me being born. I'm confused.