could I be pregnant?

We have been TTC for about 3.5 weeks now. My period was 6 days late and then lasted 2 days longer than normal. The last 2 days it was very light spotting which is unusual for me. My cycles are usually heavy and then just stop. I thought there was no chance I'd be pregnant because my period had come. It's been about a week since my period ended and we have still been trying and for the past few days I've been experiencing sore boobs and back pain. Yesterday I also noticed a slight pain in the lower left side of my stomach and have had very slight cramping. I took a pregnancy test today but it was negative. Is there any chance I could be pregnant?