I believe that Sherlock texts Irene about John a lot. And for some reason I like to think that Sherlock texted her after he got out of the hospital about how John saved him and that's why she texted back.
I think the same. The conversations usually goes like this:
I : Have you told him yet?
S : Told him what?
I : You dumbass.You are so pathetic.
I : Happy birthday you gay nerd.
S : Do me a favour.
I : No I can’t send John ‘I love you’ from you. That shit doesn’t work like that .
S :Send me a text on exactly *this* time.
I : ooohh playing the jealous card you nasty fucko.
S : will you do it?
I : Consider it done.
S : thank you.
I : Tell John today that you are hungry okay ? :>
S : stfu