Any advice?

So I thought I started my period yesterday afternoon, we came home from Easter lunch at his moms, I had some light pink/light brown spotting. I thought I started my period so I put in a tampon, later on in the day I took it out, just to check if I needed to change it and what not. And there was barely anything on it. And when I went to bed I put another tampon in for precautions while I slept. But this morning there was still barely anything on it. If this is my period, it's the lightest period I've ever had. Usually it's heavy my first 3 days then gradually gets lighter by day 5 and I'm usually done by day 5. I took 4 pregnancy tests before AF was supposed to show, they were all negative. My cycles are 33 days long, could I have ovulated later than I thought and still have a chance of being pregnant? My opk had a solid smiley on the 7th of March. So idk how I could have ovulated later. I'm so confused on what's happening.