I never had a regular period and a week after me and my bf first had sex I was already sick, tender breasts, bloating, etc. we had sex only a few days after my period. I took 4 pregnancy tests only 2 & 1/2 weeks after sex and they all came out positive and my period wasnt even due. I would even go up to 8 months without a period so I would say that if you have a feeling that you could be pregnant just go on ahead and take one. If it's negative see a doctor
You need to wait for 72 hours in order to do a pregnancy test after having unprotected sex. Then, the test will be able to determine if you are really pregnant through the urine or blood samples. For reference, go to http://www.webmd.com/baby/guide/pregnancy-tests.