Hi, name is Jac. 21 yrs old.
I weight 63kg height 5"3.

Before I used to gym a lot to achieve my goal weight 57kg at least. Until I got desperate of losing weight and heard about water fasting.

About a month ago I did 5 days water fasting and lost weight about 3kg in 5 days, I got addicted in doing it because whenever I go back to my normal eating habbit I felt fat and bloated which I really ddnt like.

So since then whenever I feel fat I used to not eat for 4-5days like totally not eat AT ALL. Cus I lose weight fast by not eating but whenever I eat again and crave sugars and sweets I feel like all the weight I have lost came back, so I go back on not eating for few days again then eat then stop eating for few days again.

I don't really know what's wrong with me people say that I look very different whenever they see me, somedays I look slim and all right somedays I look fat.
And what Im concern about too is that whenever I eat small piece of food I go straight to the toilet and sometimes vommit.
Now my stomach pain is always there, my back on my lower right side has a sharp pain for few days now. It hurts a lot I went to chinese massage but it ddnt help me at all. Its still painful like a sharp feeling.
Should I go to the doctor or ignore this as maybe my body is just in shock?

Lower right can be appendix, and if you are vomiting (and having diarrhea?) go to an ER today.
'Body in shock' is a meaningless term. There might be nothing wrong with you but an intestinal virus, but a ruptured appendix is not something you want to have!

Fasting is not a healthy way to lose weight. Also, fasting may help you lose a few pounds quickly, but then you body goes into starvation mode, and actually holds on to the fat that's already there, to survive.

The best way to lose weight is to eat at least 6 small meals a day. This will actually boost your metabolism, rather than destroy it, which is what you're doing now.

As for the medical issues, you'll need to see a doctor for that.