All right, I didn't know that you were focusing on a particular ISFJ. If your question is really about why a particular ISFJ would fall into category # 1, I can try to give some insight.
It's possible a lot of times for ISFJs to get paranoid when their Ne runs rampant. When that happens, sometimes we can over worry about things in an illogical manner.
This popped up some in this thread:
http://personalitycafe.com/isfj-forum-nurturers/41333-tell-me-how-make-my-isfj-hubby-happy.html
And I think this link has been helpful for me to counter-act some of those negative tendencies.
ISFJ Personal Growth
I think the thing is that ISFJs tend to need emotional consistency in our lives, and when it's not there, we start to worry. We like to know that things aren't changing, because when certain things change, it really stresses us out.
When our inferior Ne takes over, we start worrying about all of the negative possibilities. This is especially bad when we don't have access to information. When we don't know something, we'll imagine that the worst case scenario is actually the truth. We do this because we like to be prepared for everything. Things are much less stressful to us when we know that they're coming.
It's extremely stressful for me to be unexpectedly hit with something negative, especially if I have to react quickly. When that happens, I go into panic mode, because I can't adapt to it quickly. So, I always prefer to know when something bad is coming, because that way I can take my time to get ready for it and stretch out my emotions over time.
So, when a bad thing is going to happen, I'd rather know about it as early as possible so I can prepare for it.
So the problem is that if I don't know if something bad is going to happen or not, I'm stuck...I don't know whether to try to prepare for it or not. So my default position is to try to assume that the worst will happen, and to start preparing for it. This is inferior Ne in a nutshell.
However, what the ISFJ Growth link discusses (and a few other MBTI books mention as well), is that as ISFJs, we need to make sure we don't jump to these negative assumptions without any proof. We should only believe negative things if we know for sure that they're coming. Otherwise, we're over-worrying about something that probably won't ever even happen. This has happened to me a number of times.
I mention it in this thread:
http://personalitycafe.com/sjs-temperament-forum-overseers/27984-question-isfjs-istjs.html
So, it's possible that in her case, due to her past experiences, she may have trouble trusting you because she's trying to "prepare for the worst". It's possible that as an ENTP, in general you may now show consistent behavior that she craves (not necessarily in terms of the relationship, but just your behavior in general). She may fee like she can't predict what you're going to do because you're not as consistent as she is. So, she may live in a constant state of mild panic, not knowing what to expect. This leads her to always expecting the worst and not trusting you.
Basically, it's possible that her Ne has taken over because has no strong, consistent evidence to set her mind at ease. Either that, or she's so gripped by her Ne (which again, may be due to past experiences) that she's not able to take the evidence at hand and take it face value...that she's caught in a constant wave of imagining negatives.
This link discusses inferior Ne in a good bit of detail as well.
http://personalitycafe.com/isfj-articles/78380-recognizing-inferior-function-isfj.html
I'm really only taking a stab at this based on my own experiences and based on what I've read...I may be off base. But if your question is more toward why she might be more likely as an ISFJ to fall into the first category, then maybe what I've said will offer some insight.