sorry for being late but its nothing to worry about.
"its just sunday dinner sindrome"
ray ate alot and has the food + gas to cope with. its not a bad thing and wont cause a problem, i wouldnt treat with anything and if your worried just feed a little less.
this is a common problem when you have several rays together and one is very greedy. i know i had a very nice hystrix with a particulary bad " sunday dinner syndrome " that aquaman45 now looks after.
you end up feeding more so that the rest get their fair share but you get at least one that has this lump on one side where the digestive system is (like a fat belly after a big sunday lunch )
i think its good that your rays are eating so well but i dont think its something you should worry about and certainly not something you should be treating in any way other than reducing the amount of food that particular ray eats ( and i say that particular ray because reducing the amount you feed could mean that other rays dont get enough ) as well as several others that show the same signs but i do not worry as i know the digestive tract is on the side where the bump is and it will go down if you dont feed or reduse feeding. it doesnt seem to cause the rays any distress but it can look concerning. ive noticed this seems to be more of an issue with female rays however and you will notice this is not such and issue when they are pregnant and obviously larger in the same region.
good luck, im sure your ray will be fine and congrats that she is eating so well, your water must be very good !