1. DB2 does not store table and index data within the same dataset.It allocates atleast one dataset for each index that you create.
2. The VSAM file system used by Db2 does not extend infinitely and therefore the space gets used up when you have heavy insert/load activities.Therefore you must specify adequate Primary and secondary(increment) for each table and index space by specifying PRIQTY and SECQTY within your create index statement.If secondary quantity is omitted , than a a very small default size is allocated which obviously is not enough.
I think in your case for your indexspace/dataspace the secondary quantity has been omitted.