I concur, make sure that you analyze your index again.
Also, if the column contains data that is highly skewed
you'll need to create a histogram on the column. If
the data is skewed and you don't have a histogram then
Oracle will assume a flat distribution and the chances are
it may not use the index or use it inappropriately.

Just in case the syntax is...

ANALYZE INDEX index_name compute statistics;

you can also use estimate statistics sample # rows or
sample # percent. The estimate is much faster and is
usually sufficient as long as you give it enough data
to get a good sampling. If you have a pretty uniform
distribution then you can get away with a lower number.
If you have a non-uniform distribution, then increase
the number. If unsure, be safe and do a compute.

To do a histogram if your column values are highly skewed:

ANALYZE TABLE table_name compute statistics
for columns column_names SIZE number of bands.
Number of bands, from what I can gather is similar to
number of buckets. The default is 75. If you have,
say, 8 distinct values in the column then set bands =8.

ANALYZE TABLE table_name compute statistics
for columns column_names SIZE 8;


Joe