Search Results: get back with your ex-girlfriend

It's actually like this.. My exbf broke up with me last year late November. He didn't contacted me for a month until I received a message from him after that saying Merry Christmas in 25th of December and Happy New Year of 1st Jan. of course, I replied back to him wishing him the same thing. Anyways, I greeted him in his birthday and he said, I thought you forgot. To be honest, we talked rarely. In those talked there were a times he said he loves me and miss me but whenever I ask him if he wants us to be together again and give it a try the only answer I always get was he doesn't know at all. After hearing that so many times from him I didn't asked anymore and we did stop talking again, it takes (1 1/2 months starting March and April 2014).

Even we broke up there were a times he still liked my photos that I uploaded in my profile dp or wall post on my FB. Recently, I received a notification from my FB (around 1st week of April 2014) that he liked 3 of my profile photo that I recently uploaded months ago (1 photo, January and 1 photo, February ) but the weird is one of those 3 photos he liked I uploaded that was (1 photo around August or October, I guess) last year when we were still in a relationship.

To be honest, I didn't expect to hear or received any notifications / messages from him. I didn't also give any meaning when he liked my profile photo since I was just thinking he might just liked it and nothing else. I didn't received any messages from him after he liked my photos around 1st of April 2014 until I received a messages from last night, he said a message "hey". I got that message when I was already sleeping, I slept early because I was too tired since I was outside with my Aunt to help her out in her house. I replied only today around afternoon and I just only replied back with a "hey! too."

What does he thinking by sending a hey out of the blue?
What long should I wait to reply back in his message?

Thanks for the help.