Hi Shweta,
As mentioned above
you should start looking in to heap dumps. You can use JMeter to run scripts and get heap dumps at different intervals. This will help you analyse application memory consumption at different times.
Jprofile is also a great tool that can give you snapshots of memory upto the method level. Also run your weblogic with verbose option which provide memory statistics and you will be able to see the consumption of memory changing in logs as well.
Hope this helps you in some way
.
Regards,
Chandan