What Causes Delay In Periods?

Hiya, im just wondering whether you could give me a little advice. I was on the depo shot for 3months (i only had one shot), i come off it because i was spotting all the time, which i knew was normal, i should have gone back for my second shot in feburary 2010, but i didnt. it took me untill july 17th to have a period again, i had a four day period then a week later a two day one, the month after my period was on time (25th August 2010), now i have missed two cycles. i did a pregnancy test when i was one day over due and it was negative, i went to the doctors on the 5th october 2010 and she told me to take another test and just wait for my period to come, the following saturday i did another test which made me 2weeks late and it was also negative, i havent done one since and i will be neally 6weeks late now. i do have unprotected sex with my boyfriend, we have spoke about kids and both decided 'if it happens, it happens', we have been together for over 6years now. Could i be pregnant or could it be something else? thanks