When can I start testing?

I also had very sensitive breasts a few days after my (guessed) ovulation date, took a test but it was negative. I didn't think much more about it at the time, but 2 weeks later I still had symptoms, so I took another test, which was positive. In the end, it turned out my ovulation was indeed a few days earlier than my guessed date.

Normally, you can test from the moment your period is due, because that's when hCG is produced and at a concentration high enough to be detected. A test could already be positive earlier, and if you get a negative test after your expected period date (missed period day), you can repeat it a few days later to be sure if your period still doesn't start.
In your case, you could start testing from today. Exciting! Will you let me know what the result is?

Best time to start testing after 12 dpo is also hard because there's little chance it'll be positive. I had that with my son at 10 dpo it was negative and only lightly positive at 12 dpo.

With my first pregnancy, which was via IUI, the gynecologist advised testing only 16 days after the treatment. Since with IUI you know exactly when your ovulation and fertilization occurred, I always stick to that. That way, you're sure you're not testing too early, especially if implantation happens later.