What is going on?

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I just finished my period on July 2nd. Now I'm in my fertile window. I have light pink spotting, like a salmon color. I had sex on July 7th, but I wasn't bleeding, now I am. This happened after my last period as well. I was spotting. I went to the doctor and she said I didn't have any STD's or anything. I bled when I had sex after my period last month and I bled when I was being fingered last month also. Since then, I haven't had sex or done anything that involves going into my vagina until July 7th. I didn't bleed when I had sex last week. During my period this month, I had NO period cramps like I usually do and it was blood clots and it wasn't as heavy as usual. Now I'm spotting. No pain whatsoever.. What could this be?