Hi there!
I have done a couple of fasts that has been about 3 to 5 days, although I always break them beause I start feeling like my throat is getting narrow and I panic a little bit. The sensation disappears as soon as I eat something again. Short fasts between 1-3 days is never an issue.
Is this only a psychological reaction or can it be something else?
Sounds like an allergic reaction. You might need to speak to your doctor. Possibly a food you’re eating when you break your fast?
I am far from a pro but I’ve noticed if I go a long time without eating (a 24 hr fast is my max so far) I don’t get hungry but I get this pain/pressure under my upper ribs and my heart pounds. It would go away when I ate so I started wondering what it could be… A lump of salt (I use lite for the potassium too) under my toungue, washed down with smart water or mineral water, does the trick. Maybe your body is doing something strange, like mine was, to let you know you need sodium or potassium?
Look what I found:
Looks like I was on the right track but it can be caused by low magnesium! Sounds like some Epsom salt baths are in order 
It feels like throat spasms is the closest to what I am experiencing. It becomes like a like a lump in my throat, just where the males “apple” is.
However, I still suspect it may be psychological. It kind of escalates when I start panicing and gets allieviated as soon as I eat something. I also get increased heart rhythm and similar to a panic attack. It sounds so silly, and I try to tell myself every time that I shouldn’t worry, but it all just increase.
I read something that the “acidic phase” starts around that time (3-5 days)… could this have anything to do with it?
Regardless, something makes me feel this way.