Illness/Sickness: Constipated Axolotl? Advice Please!!

I've had my axolotl for around a year now, he is about 6 inches long and white with pink gills, I love him to bits!

However, recently I've noticed that his gills are very pale, and remain that way even when he is feeding, I have been doing water changes more frequently in order to try and improve the problem, and changing the position of my filter to better oxygenate the water. Other than this he was behaving perfectly normally until today where i noticed him at the bottom of the tank on his side acting disorientated and struggling slightly. There is also a dark shadow through his skin underneath his tail (near his rectum). I have since put him in a shallow container of water and he seems to have righted himself and is standing upright again. However, he has been looking like he has been trying to poo and no luck. Have I done the right thing? Any further advice would really be appreciated, I love my pet and hate to see him unwell!!

Do you have gravel in the tank, or anything else he might have swallowed?

When my lotl got constipated his gills were actually even more red, than before, and pointing forward, as he was trying to push the stuff through. And he was, yes, standing more on one side, but it was barely noticable.

I think, if he had swallowed gravel his posture would be different and you could feel or even see the stones (as he is almost transparent ) inside. He would be sitting and moving heavily ... ofc depends on the amount of stones and their size.

Fridging can be one of the solutions, but you have to be sure of the case not to make it worse.