Period or implantation bleeding?

So I had sex with my boyfriend during my cycle, once at the start and once at the end. I got my period 3 days ago and for the first two days my period was quite heavy, but today my period has gone so light that it’s barley there. I usually have medium to heavy periods and usually get a little lighter towards the end (last day). I’ve got back ache - had the day before period day 1 - my breasts felt like normal pms breast but now are okay. Should I be worried about being pregnant? I have become very worried and stressed over this tonight, please help.