Can you have a miscarriage if you think you were pregnant for a couple of days?

If you only had sex four days ago, then the symptoms you are experiencing weren't pregnancy related. If you were to become pregnant it would be about 2 weeks before you would experience symptoms. From ovulation, if pregnancy were to occur it would take 7-10 days before implantation would occur and then another 3-4 days before your body is producing enough HCG for you to experience symptoms and have a positive test. HCG is what causes pregnancy symptoms, without it you can't experience symptoms. It sounds like your period is starting.