Bleeding around ovulation?

I've been TTC for over 3 years. I've done Clomid 5 times, 2 with IUI, no success. I was told I had to do IVF. I switched to another RE who wasn't convinced I needed IVF since my cycles seemed pretty regular. On CD 12 the US showed a 24 mm follicle. They drew blood to see if I had an LH surge, which I didn't so I triggered tonight and will do IUI on Monday. Now I'm getting paranoid because now my cervical mucus is tinged with blood and I'm worried if I actually might have ovulated on my own. I bought an OPK and they've been negative. What else could cause blood tinged CM near ovulation if I'm not supposed to have ovulated yet?