Why is my boyfriend acting reckless and cold all of a sudden?

Anonymous
We have been in a relationship for 5 months, but 2 days ago he suddenly started acting like a reckless and boring one. When I texted him he replied me like "Okay. , I understand. ''
So , at the end of the day I texted him on whatsapp I said ; if you don't want to text me then , throw seen don't reply me, I won't be disturbed by it. Then he didn't text me back again. Next evening ,
I went to see him after his working hours , but he made gestures that ignored me when I asked him why you're acting like this? replied with anger tone ''you're talking nonsense , did you come for this '' then walked fast. I decided to go home, when I walk he called me and asked where are you like he cares , when I said I'm going home. He just said OK. and we are not talking for 2 days.
Why is he acting like this? What should I do?
Why is my boyfriend acting reckless and cold all of a sudden?
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