How can u tell the difference

So how can you tell the difference if your gaining weight in your stomach area or your pregnant I hope I’m not since I’ve had my period these last two months also what’s the difference and how can u tell the difference between implantation blood and period blood what makes them so different and when do u get implantation I ask this because because me and my boyfriend and sex June 16 and I had my period on the 21st of June and it lasted 7 days which is normal and last month I had my period on the 17th and I lasted 5 also normal and I’m suppose to get my August period in 6 days