Contraceptive implant , irregular periods

In August 2009, I got the contraceptive implant inserted in my arm. A side effect of the implant is irregular periods (generally returning to normal within 6 months). I did experience irregular period so I decided to keep track of when I got my period, how long it lasted and how heavy it was. By January 2010, my period had seemed to be falling into its regular cycle (about every 28/29 days, lasting around 5 days). In April 2010, my period was only 2 days long and nothing more then a bit of spotting. It is now July 14th and I was due for my period on the 12th.. but still nothing (4 months without a normal period).

Along with the missed period, I have also experienced a bit of nausea (after eating and when smelling strong smells), although I never did throw up. I have also experienced back pain, sore boobs (only some days), headaches, fatigue and light headedness, which are all signs of pregnancy as I was told. I have also experienced weird “pulling” feelings in my abdomen and unusual feelings when touching my belly, and I can no longer sleep on my belly as it is too uncomfortable.

I have taken many home pregnancy tests, all with a negative result. I am confused because i seem to be experiencing signs of pregnancy but the tests are saying I'm not. If the “period” in April was spotting due to the egg implanting, I have calculated myself to be around 17 weeks pregnant (if i am). I am a bit bigger then the average person so I cannot notice if I'm getting a bigger belly or not, although my belly does seem to be quite a bit firmer.

Also while i was driving home the other night i was trying to work out when i would have conceived (if i did) and came to the conclusion it would have been the 27th of march. I had sex that morning and although we used protection, we did take off the condom half way through and used the “pull out” method instead. The 27th would have been the 11th day of my cycle meaning 3 days later would have been my most fertile time of the month.

Help! Please, I'm only 17 and there's no way i can ask my mom about this!