Spotting?

So i got my first depo Aug 6th. I didnt get a period at all. Im not spotting a little. Which i was told was normal. but its like very very light pink. My sister told me it was implantion bleeding since i showed her what it looked like. I did have unprotected sex the week i had to let it become effective. I took a pregnency test and it was negative but the 2nd line was extreamly faint. is it just spotting? or what could it be?