I'm just telling you what I think.
Girl says to you, "no not really, I don't really think about you"
And you reply with "Yeah it'd be pretty weird if you did think of me a lot".
You're too passive. By agreeing with her in this case, you sound insecure. Not saying you have to be really cocky and full of yourself or anything but you could've handled it better. The message you sent basically came across as "I'm not surprised, why would a girl be thinking about me?". That's no good!
If a girl sent that to me, I woulda turned it sexual... "not even in the shower? O_O". Or made her feel guilty and see if she'd apologise, ie. "you sure know how to make a man feel wanted

". A girl would do the same to you... it's playful though, remember that. That's why the winky face was invented.
On the other hand, I wouldn't be asking if she was thinking about me in the first place. I'm not interested, I just assume she likes me and if she doesn't then it'll be pretty clear within a few texts. How long have you been texting this girl by the way?