I have always been exact on my period every 28 days. Last month on July 18th I was ovulating and I had sex. My period was due on August 1st and it never showed up till August 9th. During the day it was very light, I barely bled but when I woke up for work the next morning I filled up a whole pad, I stained everything and when I sat on the toilet it was clots everywhere, I was like this the whole day. The day after this, I haven't stained at all, it is just a pinkish/white discharge. I did take a pregnancy test before I got my period & it had come out negative. Can I still be pregnant or what is really going on? Please help!

In the whole realm of women and sex and periods and pregnancy, there is no always and no never.

- You could be pregnant
- You could not be pregnant
- You could have been pregnant briefly and miscarried a pea-sized fertilized egg
- You could be pregnant now

Those are the 4 scenarios for anyone who asks. It was too soon to take a test. The heavy clots, however, suggest that you are not pregnant. They can be clotty because you are just getting all the usual menstrual lining at once and happen to see it that time, or because you were building more lining for an embryo. There is no way to know now. On rare occasions a woman will appear to have a period her first month of pregnancy, but it would be a near zero chance that it would look this full.

Now about what you wrote:
'Always been exact' will eventually not be exact, and this month might have been the first time it wasn't. And it didn't 'never show up till' because it did show up. I'm not being picky - these are good concepts to understand as you age, and much will change.
You don't really know when you were ovulating unless you have sophisticated knowledge and a thermometer. Do NOT believe online apps.
You take a test 2 weeks after the first day you expected your period to start. You were too soon.

Question: are you trying to conceive?