Late period

Bec
What are the chances on conceiving on the last day of your period? Could I have been fertile then? Me and my partner had intercourse the last day of my period and not since :( lol I am now 6 days late on my period with cramping- being tired and feeling sick. I took a test at 4 days late and got a BFN when should I test again or when should I go to my doctor? Thank you