My Girlfriend Is On The Depo Shot, We Had Unprotected
No need to keep testing.
Detailed Answer:
Hello, I'm Dr. Branch, thanks for using Ask a Doctor.
Urine pregnancy tests are typically positive 2 weeks after conception (sometimes earlier) and would certainly be positive after 3 weeks. There are some rare causes of false negatives that have to do with the testing, but with multiple negative tests, I would say that effectively rules that possibility out. Especially since your girlfriend is on the Depo shot, I would say it was unlikely she would get pregnant anyway, so I see no reason to doubt these tests and I would be reassured that your girlfriend did not get pregnant from February 14 or any other time before a couple weeks ago.
I hope that helps and answers your question, please let me know if you have any other questions or doubts about that and I would be glad to discuss it with you further.