Eat to heal! The body needs more calories when healing. You may increase yours mindfully, or at least take comfort in the fact that easing up a bit on counting is not likely to do damage to your progress. Don't go overboard, of course.
If you feel up to it, try to channel the mental energy and effort you were putting into weight loss into eating to heal yourself. Read up on what's best for your body and go out of your way to eat healthy. Veggies and good quality protein, likely - or whatever provides great nutrients and micronutrients without aggravating your IBS. Get creative! Or get someone to help you, if you're not up to preparing foods for yourself. Edited to say -- if you focus on eating well, and trying to stay mindful, you're unlikely to do all your progress. Hopefully you'll maintain or only blip up a bit (minus water flux). Focusing on eating well will help you get back to fighting form the fastest, so you can get on with your goals.
The scale will be a big old liar until you heal and the medication drops off. Water weight just goes nuts with both those things in play. Try to make sure you drink extra water to support all the work your body's engaged in.
I hope you feel better soon!