INTPs aren't emotionless, per se, we just have a significantly different means of dealing with them than most.
Not knowing this particular girl, I can't really say for certain, but there is somewhat of a high probability that she might be holding back certain things.
The best suggestion I have for you is to ask her questions along the lines of (when they can be worked into the conversation appropriately and non-nonchalantly)...
"How are you feeling?"
"Is anything on your mind?"
"Did you do anything interesting today?"
"What do you think of..."
It's probably safe to say, that the brevity of her answers is inversely proportional to how much she cares to give them.
If she tends to give you the same generic answers, I'd say you're in for trouble...
Whenever I would ask my old INTP any of the above type questions, I'd always get the same unfullfulling answers... "Good." "Fine." "Nothing." "No." "I don't know."
So, if your INTP does this, she may legitimately have trouble expressing herself, or, she may just not care enough to tell you certain things, as unkind as that may sound.
She has emotions, no doubt, it's just a matter of if she is willing to talk about them with you or not.
Best of luck to you, and I hope it turns out a lot better than it did for me.