Popular question: When did I first know?

I am guessing I was so physically and mentally exhausted the first year, that I only probably focused on the basics like changing diapers, feeding him, etc.....and GETTING SLEEP, and the fact, I was a first time parent, that you could say "normal" was sketchy at best, or what I knew about it.  I knew what I read in books and he hit all the physical milestones JUST FINE....he was so strong too.  I felt like he was a very engaging infant until he got to be about 18 months old.  We played peek a boo around his first birthday and he would giggle.  I can't say anything seemed "off" until his language was supposed to develop.  He started off with some cute baby words.  I remember him saying "Ba" for several things, like Bath, Ball, Bottle....and only I knew which one he was indicating with that same syllable.  Something changed around 18 months.  He just regressed.  The little language he had stopped.  We would call his name and he would not respond.  The eye contact was fleeting.  Other caregivers, they started to express their concerns because he would not engage with other kids.  Instead, he would go to a dark, cold corner, underneath a table and hide himself.  I was surprised he did this.  I had no answers for it.  I had no clue why his behavior seem to change overnight.  These are just examples, but I started to know about that time frame that something was just not quite right.