How many high days is normal?

I just started using the advanced fertility monitor this month and was really surprised as I tested low on day 6 and then high today on day 7. I test first thing in the morning with very concentrated urine. I have a pretty regular 28 day cycle and am fairly sure I ovulate on about day 13/14 due to previous ovulation tests I've used and usually I can feel it too! I'm a bit concerned as starting high days on day 7 implies I'm going to be ovulating really early maybe even day 10, much earlier than I thought, which I know isn't great news for conception. Is it possible to have more than 3 high days before ovulation? Could the test have just picked up te estrogen rise early? Thanks!