Is spotting for the first time a sign of pregnancy?

Hi doctor,

I am on the pill Lo Loestrin, but I have not been consistently taking it. My last period was on the 25th of last month and my average cycle is around 28 days. My husband and I had unprotected sex on 5th and 6th. I noticed some dark brown discharge on my underwear. When I wiped, it was mixed dark brown and red strands of blood. It has been more than 24 hours and very light when I wipe. I also have been experiencing soreness and light cramps as I would feel in the beginning of my period. However, the discomfort in my abdomen is located on one side and it comes and goes. I have soreness in my legs, lower back and breasts. I also have been experiencing some aching sensation in my vagina. I have also experienced sudden feelings of nausea, but it has not lasted very long. When I began spotting, I was very surprised. I have never spotted between periods before. I am not sure if this is because I have missed taking my pills or if it is implantation bleeding. I have been driving myself crazy by thinking that I could actually be pregnant. The thought of having a baby makes me so excited. But, I do not want to get my hopes up. Please clarify.