took 3g ascorbic acid,may this cause negative HPT?

Hi,
Welcome to ehealthforum,
If you have unprotected intercourse closer to your fertile days or probable time of egg release, then chances of pregnancy cannot be ruled out. Pre-cum, or pre-ejaculatory fluid is known to contain sperms in it. Sperms once released inside female reproductive tract can stay alive for upto 3-4 days. The egg once released from ovary can stay alive for 6-7 days.
Your current symptoms (delayed periods, breast tenderness, low backpain, abdominal cramps) could suggestive of possibility of early pregnancy. If your periods are more than 10 days late, you should consider visiting your doctor/gynecologist for further evaluation/examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately). Drink plenty of water. Take adequate rest. Maintain healthy diet.
Urine pregnancy tests are not always 100% reliable. Sometimes they can also keep showing you false negative results during the first few months. Urine pregnancy test kits are calibrated ones, so they show positive results only after the hormonal levels in urine have reached a certain level. Blood/serum HCG levels are accurate and can detect pregnancy early (as early as 11 days post conception).
Take care.