Late Period

Based on my app I'm 8 days late as of today... Took a pregnancy test on Saturday one on Monday another one Yesterday morning and all have been negative! Not sure what is going on.. I did take plan B back in April and got my period twice not sure if it still affects it.. last period was 4 days late so not sure if it's ok. Tbh don't want a baby due to I have a seven month old... My husband and I used condoms so we are confused on what's happening.. I can say i've been over stress recently not sure if that can be it? any advice