First workout since flare up and today I was down and spent the day in bed

I had a pretty bad flare up from July to December.. recovered slowly but did well so on Monday I did my first workout with a trainer, she herself has an autoimmune disease and knows that I need to watch to not overdo it.. the workout felt good, I felt ok on Tuesday but yesterday my leg started hurting and today I had fatigue, was freezing despite the apartment being 80 and felt like I would throw up any moment.

The workout was a light one, body weight, slow and steady but now I am worried.

What is the best way to avoid feeling like this after a workout? I miss most my workouts from before my first flare up and really want to get back into it but how do I do this without these days?