I’m an OB/GYN nurse practitioner, so I deal with this question frequently.
Modern urine pregnancy tests are quite sensitive and accurate if instructions are followed to the letter. The urine test we use here in the office will be positive at 25 mIU, and will pick up most pregnancies by about 2 weeks after conception. Home tests vary – the most sensitive appears to be First Response Early Result, whivh is sensitive to about 7mIU, similar to a serum blood pregnancy test.
Ultrasounds can’t reliably diagnose pregnancy until 3–4 weeks after conception.
If you have not had unprotected intercourse in the last 2–3 weeks, and you’ve had 2 negative pregnancy tests a few days apart that were done correctly, it’s very unlikely that you’re pregnant. If it’s unusual for you to skip a period, probably good to see someone, and if you’re having lower abdominal pain, do it urgently.