Customer support can tell you what days you used data but not what it was used for. They can give you a rough idea of what it was used for based on the amount used, but not exactly. The best idea would be to talk to someone from customer service that can look up your account for when data was used.
One thing that you may be interested in is putting a data block on your line. This would mean that no data could be used, but it would also mean that you could not send or receive picture messages either.