Hi
Of course you could be pregnant. If you have unprotected penis-vagina sex, you're putting yourself in danger of pregnancy. There is no safe time to have unprotected p-v sex.
Your period is due 28 days after your last period, so do the math. You are overdue.
I suggest you use another pregnancy test kit. If the test is still negative but you continue to have symptoms of pregnancy, go to your local women's clinic or Planned Parenthood for a free confidential test with a higher reliability than the ones you can buy. Dr. J
As far as I know most pregnancy tests aren't reliabl until a week after your missed period. And if the test says you are pregnant it's accurate but if it says negative there is a higher chance that it's wrong.
You put yourself at greatest risk by letting him ejaculate in you, if you came with him I would put money on you being pregnant.
But these are all opinions from my experience and not related to medical fact in any way.