Hello zeeth. Beautiful fish you have. Love the whiptails. To really know whats going on we will need more info for sure.
How big is the tank and how many fish are in it?
Tank temperature ?
How old is the fish?
What do you feed it and how often? Maybe its just bloated from food, these guys can get bloated from too much protein and meaty foods. They benefit from plant matter in their diet. Although since you said you havent fed it for a few days in would be worried its stomach hasn't went down.
This makes me think bacterial or parasite off the bat.
If your tank water is in a bad state thats gonna cause fish sickness most the time because it weakens their immune system when their environment isnt ideal. For example if its too cold or not clean/overcrowded.
What filter do you have on it?
Also a top down picture will let us know gender. Males more rounded head while females are thinner and more pointed. Just from that pic I would guess male.
Tell us more about the fish behavior and appearance. Is it letharagic? Does it swim properly? Any redness? Bulging eyes? You know best because you have observed your fish. If anything seems off let us know. Is it hiding? Flashing? which is sudden quick movements like its scratching itself on something. Is it pooping and what color? White poop is bad. Red worms coming from the anus? Fish are similar to us if you are sick and feeling bad you will show it. Its similar with fish. Just take some time and observe the fish. Any clamped fins? Things like this.
If it was my fish I would use a bacterial med like
maracyn or maybe canaplex. If you have it a antiparasite med wouldn't hurt like
fritz para cleanse. For
antibacterial/ antibiotic something with clyndamycin(maracyn),
kanamycin(kanaplex), or metranidazole.
You can use all these meds together usually but check before you throw them all in at once. Treating a lot of diseases is a guessing game a lot of times because we can only go by what we see. Unless you had a microscope for fecal samples to look at any worms or parasites in the gut.
If I didnt have any meds and I had about 50 dollars I would go for these meds just to make things easy. Fritz maracyn and para cleanse and aquarium solutions
ich x. The latter for fungal and ich. If your local store has these even better and cheaper.
Hope this helped zeeth and I hope your whiptail pulls through. We gotta figure out if he is indeed sick tho. Let us know zeeth.