Menstruation :: Brown Discharge - Implantation Bleeding?

My gf period was from the 16th to the 20th. Two days after she wanted to have unprotected sex, she put it in, but literally 5 seconds after I made her get up. I didn't ***. And I know precum can have sperm if I ejaculated earlier and didn't go pee before sex. But I know for sure I went to the bathroom after my last ejaculation because it was probably a couple days ago. Anyway. She ovulated on the 29th; The 14th day of her cycle. I think this was the day because she had light cramps and was craving sex. On the 2nd of June she has bleeding. A brown discharge. It was enough that she would have used a tampon for, not spotting. At first I was scared of implantation bleeding but it says on average is happens 9 days after ovulation with a range of 6 to 12. It had only been 4 days. I know she has been stressing, and she tried weed for the first time recently. But could that really cause this discharge. After considering all these factors, even if my precum did have sperm it's a very small chance that the sperm got to the egg with having to last 7 whole days to ovulation. I know ANYTHING IS POSSIBLE, but Its just not adding up to be implantation bleeding. What else could this be?