Unusual dog biting/scratching

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I have a 14-week old Corgi and all seems healthy except recently I noticed a few things.

1. He sits down A LOT and when he does he leans more toward one side in comparison to the other.
2. Tonight has been the second night in a few weeks where he's aggressively going toward his rear end area (kind of how dogs chase their tails). The main difference here is he does it a few times then stops and cries then attempts again.
3. He seems extremely uncomfortable, he tries to lay down, then he's woken within a minute with this issue and begins to bite toward his butt area mostly only his right side.
4. No signs of fleas, ticks, or any other illness. He is being treated currently for Giardia, but he's nearing the end of his medication.

Any advice would be appreciated. We are also taking him to the vet tomorrow, but just trying to see if anyone else has dealt with this..

-Also, I am not sure if it's common but when I lay next to him, his stomach is very rumbly.