(Potential) newbie here – with a lot of questions

Hello everyone, I (M24) had an infection (didn't test for COVID) around 7 months ago that left me with pneumonia and fatigue in the following months. I rested quite a lot since I went jobless and delayed my studies for the time being. After I got clearance from my doctors, I went back to a more active lifestyle, including a normal amount of steps and even the gym around 4x a week. Though I still had some fatigue and migraines now and then, I thought I had recovered around 80% from my long COVID/ post infectious symptoms… until now. So far I didn’t really think of ME/CFS, since I couldn’t clearly say that I had PEM after exertion, just overall weaker regenerative capabilities and a base level of fatigue. 7–8 days ago I worked out like usual, just with a bit of tiredness in my system, and I suspect this could have triggered my first real (rolling) crash. I developed flu-like symptoms (muscle aches and sore throat) as well as POTS-like heart rate anomalies. I started using the Visible app and it is kind of confirming that my HR and HRV are decreasing day by day, even though I rest more than I probably ever have. I would like to get some feedback and evaluations on my situation, since I haven’t really read many cases where someone was kind of in remission from COVID (or whatever) and then started developing PEM. I would also appreciate some tips for stopping this first PEM crash and avoiding further damage. How would you guys go forward? What would you supplement, which doctor should I seek for a potential diagnosis (Germany-specific info would be great), and how should I act in the next two months while I’m away from my studies (radical rest?) Also, some positive feedback on what other health issues can cause PEM-like symptoms would be helpful. And what would you wish you had known at the beginning of this illness? If you read this far, thank you very much...really, every kind of input is appreciated :)