My ex texted me happy birthday yesterday and I felt strange because when we were together she never even wished me before as she said she's too busy to remember anything.

Fyi: she broke up with me last year on my birthday... how ironic is that LOL!!

Below is the text and duration of my response:

Ex : happy birthday. Hope you had a good one (11.34am)

Me: tq (2.00pm)

Ex : so how are you? (2.45pm)

Me : hey am all good. Thanks. (5.45pm)

Ex: nothing to chat about? (7.30pm)

And it ended there. I am not sure if I want to reply or not because I just don't know how to response to that the most diplomatic manner. Any advise? And just curious about such behaviour from the men's and ladies' point of view.

What message would you want to convey with a response? How do you feel about her contact? I would have left it at thank you and gone about my business.

I personally think you handled it very well. When someone breaks up with you, why would you jump right back into being all friendly like nothing ever happened?
She may have just been dumped, is what I'm guessing. Or been through a string of lousy relationships, and all of a sudden you are looking good to her.
BUT she might just be bored and you are filling in while she looks for someone new to torture.
Or her boyfriend of the moment is away and can't talk much so she is just contacting you and everybody to keep her ego going.
You know her better than we do.
On the spectrum of 'just filling in' to 'suddenly you look good again' where do you think she is? I imagine that you don't know either, so speculation is wasted thought.

Regardless, don't forget that she broke up for a reason, and that isn't likely to be any different now.

You did the right thing. Any more chatting could be an opening for future contact. I'd leave it at that.