I had my last period 3 weeks, but 4 weeks ago I had unprotected sex . Could I be pregnant ? I have much feeling in the chest , with a little pain and a lot of headache . I took a pregnancy test yesterday and was negative .

If you only had sex 4 weeks ago (there has been no sex since then) then had a normal period 3 weeks ago, it is unlikely that you are pregnant. You can always take a pregnancy test to be sure. Since you have more than 4 weeks since you had sex, her pregnancy test is 97% accurate. If you have been having sex (with protection) after the last period of 3 weeks, pregnancy is a possibility. It may have become pregnant after the last period and then test yesterday was too early, if you do not get your period the following month should be another test.