How does my ex know about this?

I broke up with my ex 1 year ago
But we kept in touch nearly every day until last month
I was the one who broke up with him because I felt like he was immature (he's 2 years younger than me)
We were together for 4 years
But I still love him so I always maintain contact with him nearly everyday

Even though I have been dating a new guy for few months now
But I don't put pictures of me and him on any of my social media
So my ex boyfriend doesn't know I'm dating someone

2 months ago, he told me he got a job at the Airport
I work at the Airport too (for nearly 3 years now)

Now that he works there, sometimes we see each other randomly

But 1 month ago, I felt like I was the one always making effort to talk to him
So I didn't message him for 1 week
Then he deleted me off ALL social media (facebook and snapchat)

He didn't even tell me why

So on New years day, I texted him "happy new year"

Then he replied:
"Hope you and your boyfriend have a happy new year"

How the heck does he know about my boyfriend?
I never showed any pictures of my boyfriend on my social media!
And my boyfriend never picks me up from the airport
How does my ex know about this?
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