Is there any way I could still be pregnant…?

Patient: I’m 20 years old. I have unprotected sex a day after my four day period. Close to 20hrs later I took the morning after pill. A week and a half after I had unprotected sex I made a dr appointment and they did a urine pregnancy test, which came back negative. Yesterday would be three weeks since I had sex. I have nauseous feelings (not to strong) that comes and goes mostly after I eat, but no vomiting. Then yesterday I noticed that my nipples were getting sore. My lower back also hurts at random times and yesterday I started feeling light cramps in my stomach. Is there any way the dr’s pregnancy test was to early to tell and I could still be pregnant? Or are they pretty spot on? I know that the morning after pill might make me late for my next period so if it doesn’t show next week I shouldn’t be worried. My next period is due on the 21st of this month. Any advice would be great.