18F, I have lots of small white hairs with black bottoms falling out - why?

It sounds like you're noticing a mix of shedding and regrowth, which can be common after a period of telogen effluvium (TE). The small white hairs with black bottoms that you're seeing are likely telogen hairs. When hair enters the telogen (resting) phase, it can lose pigment and appear thinner or lighter in color before eventually shedding. The black bottom may be the root or bulb, indicating that the hair is being shed from the scalp.
Since TE causes more hair to enter the shedding phase, it’s normal to see a variety of hair types falling out, including shorter regrowing hairs (which can look thinner or white) and longer, thicker hairs. After TE stops, it takes time for hair cycles to return to normal, and some regrowing hairs may fall out as new cycles start. This can look alarming but is usually part of the recovery process.