Does Light Brown Vaginal Spotting Indicate Implantation Bleeding?

so okay... first off i was on the depo prevaera shot Januraury of 2010 right after i had an abortion i was 15 at the time.. me and my husband have been trying to concieve... and i mean reallllly trying hard to concieve... an normally im supposed to start my period today. instead i started light spotting last night and all day its been very little dark blood mostly brown... and i was wondering if it may be implantation bleedinging or if its just my period normal ....