late period... maybe implantation bleed 4 negative tests am i nutz?

Hi, I am writing because I had a really weird cycle this last month. My last period was June 18 2011. On the 3rd of July I started having some strange light pink spotting only when I wiped. It lasted almost three days and then it turned brown and stopped. I haven't gotten my period. I was feeling pregnancy symptoms for a week or so and then once I took a test and it came out negative the symptoms seemed to go away. I have taken a total of four tests- all negative. I think I took the first three too early I don't know they were the cheap kind. But I got a first response early hpt yesterday and took it in the late afternoon at the drug store when I didn't really need to go but I made myself go anyway.... and it was also negative.
Why the bleeding? I never had that in my life! What is it? I've heard of implantation bleeding but I'm not all that convinced of it... since I never had it, I don't know what to think... I am 36 years old and have had 3 successful pregnancy's. The last one being 10 years ago. Could I still be pregnant???  Or is something else wrong? Can certain medications affect hcg levels? Help... I don't know whats happening... I have been feeling really moody, weepy and been having very vivid dreams, boobs aren't sore but seem maybe slighly bigger.... but then again I'm getting fatter could just be the weight gain.. I've put on weight a bit but I think it could all be in my head. I don't think I'm pregnant but I also don't think my period is coming. Any advice?? PLEASE HELP!!!!  This is really weird for me and scary.