Week after period ended, condom slipped off inside me. no sperm. only pre-cum. took plan b one and a half days later.

Should be ok: Should be Ok. Take pregnancy test if you miss your next period.

Risk.: Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. Pre-ejaculate sometimes contains viable sperm. Risk is not very high, but is still present.