confused

Hi, i got a normal period late june and a light one early july. Since then i havent even spotted, ive had several symptoms which last for a day or two for example nausea, headache and lower back pain. I took a first response test last month and one last week which both came out negative. For the past few days my nipples have been sore and i got slight cramps. Im still thinking its my period on the way but my friend suggested that i go to the doctor. Has anyone experienced this and found out they were pregnant later on? Any suggestions?