Scared I'm pregnant

Had protected sex around April 28th and a few days after that. We used a condom each time and it did not break or leak, we both were both very cautious. My period is due on the 19th but I woke up this morning and my nipples were a little sore. I took 2 first response pregnancy tests a few days ago and they were both negative but I'm still afraid that I could be pregnant. I have no other symptoms except my stomach feels weird. I am so nervous that I could be pregnant, please help me