Thank you for reaching out to the EmpowHER community about your question. There is no chance of a pregnancy here.
1. You wiped away the sperm with a tissue.
2. You washed it off with cold water which will kill sperm.
3. If any did happen to remain on your skin, it would certainly have died before 15 minutes.
Anon1337, to reiterate and confirm your question, cold water is enough to kill sperm. Once sperm is DRY it is dead and cannot get someone pregnant.
Your girlfriend will be just fine.
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