Pregnant after period?!

Ge

Hi guys, I’m so confused. I took a pregnancy test the morning my period was due on the 18th and had a very faint line. That evening I got my period, pretty heavy but nothing out of the ordinary. Started feeling nauseous and threw up which is super unlike me so on the 22nd so took another test and it was faint but darker than the previous, and the same again this morning. I’m guessing it’s a chemical pregnancy but just confused as to why the lines seem slightly darker!