3 days late

So my period was supposed to come Thursday and I still have not got anything yet. I was having cramps and lower back pain, thinking I was going to start but nothing came. I took a home pregnancy test today and it came back negative. My cycle is always on time and never late. Some people told me that a test wouldn't be positive until 5 days late. I have a appointment Tuesday for a blood test. Any other ladies experienced something like this?