Chemical pregnancy?!

A few days after I was supposed to start my period I began spotting. I spotted for a week so I thought this was just a light period. But then the week after I had a full on heavy period with clots. Then I had a week of nothing, until yesterday I noticed a little blood and then today I began another full on period. 
I have been having sex and I am not on birth control. My boyfriend does however use condoms,although we have been putting them on half way through which I understand isn't the best idea if you aren't trying to get pregnant. 
My period is always regular and i can't think of any reason I would be experiencing a week of spotting and two weeks of period in a month when I'm not taking any hormones or changing diet or exercise or under stress.
In any case, do you think I had a chemical pregnancy? What are your thoughts and own personal experience?