Hi, and welcome, !
First off, I hope your stay in the mental health facility was helpful and that you were able to feel supported in your journey towards better mental health. That said, I know many meds used to manage various mental health issues can cause digestive upset and a lack of appetite. I know you have discontinued them, but it's something to consider. I have a family member that has been on many medications like these and some of the symptoms you described sounded pretty familiar. Ambien can cause feelings of nausea, so if you are still losing weight or feeling nauseous, that could be a culprit.
Also, I am sure you know how our mental health can directly impact our physical health in many ways. Please know I am not your doctor and I cannot provide medical advice or diagnosis, for your safety. But, your symptoms don't sound much like a relapse to me. I would talk with your doctor and your therapist, but many of your symptoms could be stemming from the anxiety and depression and/or the medications used to treat them. Depression can cause appetite changes, sever fatigue or feeling like life is 'bleh', etc. Anxiety symptoms can include palpitations, digestive upset, and more -- https://www.who.int/news-room/fact-sheets/detail/anxiety-disorders. And, your body may have been in overdrive as it fought off and then recovered from a serious episode of the flu, too.
Also, please keep in mind that your can experience comorbidities with MS, including IBS. You may want to check with your doctor about whether or not you are dealing with IBS or some other bowel condition. You can read a bit about digestive issues and MS here -- https://multiplesclerosis.net/living-with-ms/gut-punch.
It can be hard to pinpoint exactly what is causing these issues, but, it doesn't sound like a MS relapse. But, again, your doctor would be best suited to answer this question. And I would definitely keep talking to your doctor about getting some relief from the heartburn, acid reflux, etc. There's no need for you to keep suffering with these issues!
I hope that overall, you are doing okay and feeling ok, both mentally and physically.
Best, Erin, MultipleSclerosis.net Team Member.