Can I trust the negatives?

My girlfriend and I had a close call. 14 days after the incident, she took a test. They were both negative. 21 days after, she took another test. It was also negative.

We then had another close call.

14 days after the second incident, she took another test. It also was negative. They're the pink easyquik tests you can get from dollar general if it matters.

The reason I'm asking is she still hasnt gotten her period, which is freaking me out.

Also, she took a Plan B on February 7th, and her last period came like a week early. Could the pill be making her period be unusually late this month?