Hi Sarah,
It could be a number of things:
a) the pitch your tuning to is too high for the depth of the drum
b) un-even tuning of the tuning rods
c) rough edges to your bass hoop.
Although I haven't seen it - I think c) is probably the most likely answer. Next time you take your skin off, run your fingers round the edge of the shell and the hoop to check there aren't any sharp edges or splinters. If there are, a gentle sanding of the rough area should help.
Drummer Bummer
p.s. be careful with your fingers!
Drummer Bummer, November 2008