I know very slim chance of pregnancy... but I have to tell someone

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My cycle is pretty short, usually about 25 days or so. Last Friday I had brown spotting, which is a very typical start of my period, so I put in my menstrual cup and figured my period was here. However... it hasn't come and now it's been 3-4 days. The spotting is also done and I have no cramps, but do feel like I have to pee a lot. Last Friday it was 20 days since the start of my last period, which makes me think it's not mid-cycle spotting (which I don't think I've ever had). My partner and I use condoms but did some PIV without ejaculation last month - according to some online calculator, exactly during my fertile days. Not exactly huge odds, I know...

What do you think, is there any chance of pregnancy?? so the spotting would have been implantation bleeding? When can I start to reliably take a pregnancy test? If it IS pregnancy (I'd want to keep it), any vitamins or anything that I should start thinking about right now??