I just metered in my house blinds up, mid day, sunny outside, meter at floor. I bet if you wanted to photograph inside during the day with 100 speed film and a f8 lens, you would need about a 1 second exposure, ie, hold down the shutter for one second while in B mode. Were you previously shooting indoor in 1/100 shutter mode? That would explain the under exposure.
I highly suggest a book by Henry Horenstein - Black and White Photography. It will really help you get all this down, ie the photographic triangle (shutter, film speed and aperture)
Keep trying and keep asking questions! We want you to succeed in film.