I've been on it
Quick question...
We know that when we say "I'm on it" we mean that we either are in the middle of something that somebody asked us about, or we are about to start, imminently. At least that's the idea that I've always used it/heard it.
Now, I've just written an email in which at one point I said "Regarding blablabla during this past week, blablaba. But I've been on it."

Now I'm am re-reading it and it begs me the question...Hold on..Maybe "to be on it" loses its meaning if it's not in the present tense...But does it?
So here I am...What can you guys tell me about it?

By the way, any mistakes I made in my writting, please, by all means, let me know.

Question

I've been on it
Quick question...
We know that when we say "I'm on it" we mean that we either are in the middle of something that somebody asked us about, or we are about to start, imminently. At least that's the idea that I've always used it/heard it.
Now, I've just written an email in which at one point I said "Regarding blablabla during this past week, blablaba. But I've been on it."

Now I'm am re-reading it and it begs me the question...Hold on..Maybe "to be on it" loses its meaning if it's not in the present tense...But does it?
So here I am...What can you guys tell me about it?

By the way, any mistakes I made in my writting, please, by all means, let me know.

Answer

"Regarding blablabla during this past week, blablaba. But I've been on it."

I agree with Neil. This is fine to say and your meaning is clear, but it is informal.   You could say, "I've been working on it," to make it a little more formal.

Answer

Cheers Neil. 
I had already interchanged several messages with the same recepient, so the tone was not that formal eventually. So I think it was just fine, in that sense.

Answer

This makes sense to me. I would understand the meaning if I received an email saying this. It is quite informal for a written text, especially a business email. This would be my main concern about using this expression in the context you describe.