2 months ago I had pneumonia. After I stopped taking meds, I suffered from tremendous hair loss. What is the reason behind it?

You'll want to sit down with a physician to review your situation in detail. There are actually quite a few possibilities for your hair loss. Some individuals develop hair loss after any illness. We can this "telogen effluvium."  A telogen effluvium can improve in 6-9 months and often can resolve completely (provided the trigger that caused it in the first place has resolved). However, there are other causes of hair loss that need to be considered and sometimes a massive shedding can speed up the age at which an individual develops genetic hair loss. We call this "accelerated follicular miniaturization from prolonged shedding (AFMPS)," Please see a physician to review your story.

Illness can induce hair loss and in men with genetic tendencies an illness can bring on genetic balding