Would a pregnancy test be accurate 3 months after last having sex?

I had 2 periods that were short and light after having sex in June which is a little unusual for me. This month, my period decided not to show up. It's been 40 days since my last period

The sex I had in June was protected (spermicide and condom) AND I took plan B a few hours later just in case.

3 months later, I'm still worried I could be pregnant. I took a test yesterday morning and it was negative. Could it be a false negative especially since I haven't been this late for my period in a long time

Please help? I'd consider suicide if I was pregnant with my ex's baby
Would a pregnancy test be accurate 3 months after last having sex?
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