Quit Finasteride after 3 weeks because of sides - why is my hair shedding like crazy?

It’s not uncommon to experience a shedding phase when starting or stopping finasteride. What you're describing sounds like a classic "shedding" reaction, which is a temporary phase where the hair enters the telogen (resting) phase before falling out and making room for new growth. This can happen as the body adjusts to changes in the hormonal environment caused by finasteride.
Shedding typically starts within the first few weeks to months of treatment and can persist for a short while after stopping. Since you've only been off finasteride for a couple of weeks, your body is still adjusting, and it's normal to continue seeing shedding during this transition.
The good news is that the hair lost during this shedding phase is usually not permanent. The follicles should eventually produce new hairs, but it might take a few months to notice regrowth. Keep in mind that finasteride takes time to impact hair loss effectively, so stopping it may allow your previous pattern of hair thinning to continue.