If you're needing to go to the toilet for a pee more often too and only passing a small amount on each occasion, that is also suggestive of cystitis and/or a urinary tract infection so I'd also see about getting that checked if it doesn't make anything better.
Really, you need to try and stop the purging as that will even cause a LOT of problems as well as mess up your digestive system and so I'd try to get help for that as well as try and cut down on how often you do it if possible, which I know is hard but it will be better in the long run. If you haven't got a professional to discuss your purging with right now I would suggest that maybe you talk about the emotional reasons behind it with the supporters via live help/
support@recoveryourlife.com and see if they can help you in any way.
As it sounds like you may also have cystitis then try and ensure you drink water frequently and some say cranberry juice can help.
And, if may not be that so if it does continue to happen (the pain in your lower abdomen at night) for a while or worsens then you need to get checked out and even if it is a problem with your bowels there could be totally different reasons behind it than your eating problems.