I Lied About A Classmate's Birthday

I recently told my married grad student friends about a classmate’s birthday that I had seen on their social media, but the thing is, it was actually mine. I had forgotten when my actual birth date was and saw this person's post. I felt so ashamed admitting to them that I don't even know my own birth month. It seems like such a silly detail, but lying about it just made me feel worse.