INFJ or ISFJ? INFJ or INTJ?

Could someone share with me the main differences in the following types?

1) INFJ v. ISFJ

2) INFJ v. INTJ

I am pretty sure I am an INFJ, but, during moments of stress, energy conservation, or something unusual, I seem to test out as ISFJ or INTJ. For example, I have periods of time when I "withdraw" or "shut off," which is usually when I am going through a stressful social/relationship situation. During these times, I test out as an "S" and I think it's because I have a harder time trusting in my intuition and instead rely on my senses. At the same time, when I'm going through intense studying/writing for exams/papers, I test out as "T" (for obvious reasons I guess). But I always go back to INFJ. Is this normal...? I'm pretty sure that I am an INFJ especialyl when I read threads about other INFJs that have acted similarly in similar situations, but at the same time, I see parallels in yself with ISFJs and INTJs