No period while on the Depo shot?

I haven't had my period while on the depo shot. I'm 20 years old and this is my first time being on the depo shot or any form of birth control for that matter. I feel so insecure without my monthly periods to reassure me that I'm not pregnant. What do you think? Is this normal? Should I be concerned?

Here are the facts:

May 29th:
-Start of Period
-Weight: 104lbs

June 13th:
-Went to my doctor
-Negative Pregnancy Test
-Got Depo Shot #1
-Light pink spotting
-Weight: 104lbs

August 29th:
-Went to my doctor
-Negative Pregnancy Test
-Got Depo Shot #2
-Weight: 104lbs

September 30th:
-Spotting
-Weight: 104lbs

October 1st:
-Spotting
-Weight: 104lbs