If i had sex on the 3rd and my cycle is due today could i be pregnant and if so when should i test i have be having headaches nausea back pains and cr?

If your period: If you had sex on the 3rd and your period is due today, that means you had sex in your most fertile time. If your period doesn't start, check a pregnancy test tommorow and, if it is negative, repeat it in 3-5 days to be sure. Your headaches, nausea, backache, and cramps could be due to pregnancy or could be something else. Hope this helps.