This is a way to describe how you've acted for a few days or months that's different from how you were in the past. When you describe yourself in this way, it sounds like this is a characteristic that isn't usual for you.
You can use a simple adjective like "tired":
I've been pretty tired lately.
Or you can use a longer phrase like "in my own little world" in this example:
Sorry, I've kind of been in my own little world lately.
The phrase you use should be similar to an adjective, meaning that it should describe the noun ("I" in this case).
There's a little confusion about the difference between "recently" and "lately". "Lately" is more appropriate for continuing situations, while "recently" is better for events that happened just once, a couple of times, or that have continued but are finished now:
Have you been going to the gym a lot lately? You look great!
I ran into Stacy recently at the Food Emporium.