Pregnant, wondering what is happening?

So I ended what I thought was my period... normal like a period, and 3 days later started light spotting, light pink dark brown, mostly on the toilet paper when use bathroom. Sometimes, rarely, it looks like there may be a TINY piece of tissue.. this had been going on for almost 3 weeks. Turns out I am pregnant. Went to doctor, they said spotting and maybe tissue pieces can be old remnants of sex, old in general or implantation... and my period may have been implantation even tho it was heavy. I have no cramps, my boobs kinda feel tender, sometimes alittle more than others. At one point for one day I had very mild pressure low uterus area for an hour or 2, thats it. going for blood work tomorrow, so Im either about 4 weeks or going on 8 weeks. Blood work will tell more. jw if anything dollar has happened to anyone else?? thanks.