Because you've had sex, it is possible that you're pregnant, but if you used protection, especially a condom that was used properly, the chances are slimmer.
If your symptoms are indeed due to pregnancy, then you could start testing about four days before your expected period and likely get an accurate result. If it's negative the first time, wait two more days and test again. Continue testing this way until your period arrives or you get a positive test.
If your symptoms aren't due to pregnancy (which again, you're probably not pregnant if you used a condom properly each time), then they could be from ovulation, which can cause many of these symptoms, especially bloating and nausea, or, as you mentioned, PMS.
Another possible cause of your symptoms is a urinary tract infection (UTI), especially because you mention back pain and frequent urination. Having sex every day for six days could have exposed you to bacteria that would cause a UTI. Keep an eye out for fever, pain with urination or cloudy/foul smelling urine and if you notice any of these symptoms get into the doctor. They can test for a UTI the same way they test for pregnancy - with a simple urine test. You could even request both tests at the same time.