18 DPO with negative test

As the title says, I'm 18 DPO and I just took a test about an hour ago which came back negative. Stress levels have been higher than normal but nothing that I haven't experienced before, I doubt it is the reason for no AF. I was supposed to get my period on Sunday the 23rd but nada. I plan on taking a test again in the morning.
​I used First Response, which is what I've always used and have been trying on and off since October. Any advice? I also can't get in to see a doctor for blood work currently as I have low money because of unexpected work on my car. It's a weird situation, I know, but it'd be nice to have some sort of advice or reassurance that I'm not driving myself crazy. Thanks!