Spotting before period? Any advise would be great! :)

HI, I'm 24 and I was just wondering if anyone could answer a question for me. My husband and I have been trying to have a baby but not exactly trying. We just dont use protection and say we'll have one when God is ready for us to concieve. We always try to have sex about every other day a week after my period stops until its the time for it to come again. Anyways I just started taking adipex(diet pill) 3 weeks ago and I was supposed to start my period on Nov. 20 and have not. But last week on Wednesday night (Nov. 15) I thought my period was coming early because I started very lightly spotting. It lasted till about Saturday morning (Nov 18th ). So light it didnt even show up on a pad or anything. So I kept feeling like I was about to start 'really bleeding' and never did. I had light cramping, but nothing like normal. Now its 2 days past when I normally start and nothing has happened. So my question is was that implantion bleeding that some women get. OR do you think it had something to do with me starting taking adipex. It is possible I could be pregnant, but I'm not having any symptoms now. Not even breast tenderness which I normally have with my period. But nothing except for some dizzyness. And if it is not that im pregnant, than what else could cause the light spotting with no period following it? Also I have taken 2 test, both neg. Any advise would be great! Thanks everyone!! :)