It seems your database is crashing. Before driving to that conclusion, see the logs.
See your db2diag.log file for more information. To locate that file, use the command
db2 get dbm cfg |grep DIAGPATH
Also,
Pls psot the contenets [last 20 lines] of
1. db2diag.log file in this directory
2. 20 lines of Notify log : It will be like <instance-name>.nfy .
3. Conents of DIAGPATH directory.
4. DB2 Version, Fixpack, Operating System Version.
These will help to analyze.