Did your partner ejaculate before the condom slipped?
Did the condom actually come off, or did it just slide down a little, or twist a little?
If the answer is yes to the first question, and the condom either came off or slipped a great deal, than you may have a small risk.
If not, then you don't have anything to worry about.
Condoms slipping usually happens for two reasons: nobody holds the base of the condom when pulling out, and/or the condoms are too big. So you might want to think about condom fit, as I'm guessing that's your problem here.
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