can drinking alot of water to go pee effect test results?

Hi,
Welcome to ehealthforum,
Most urine pregnancy test kits usually start showing positive results after about 20 days post-conception (roughly corresponding to about a week after your missed periods). You can also consider taking a urine pregnancy test (on an early morning first urine sample) after 7-10 days to see if you get positive results, or get it confirmed by a blood HCG test (avoid lot of fluid intake or caffeinated beverage intake in the previous night before test). Wait and watch for your periods. The symptoms that you describe (delayed period, abdominal discomfort, headache, back pain) could also be suggestive of possibility of pregnancy. You should consider visiting your gynecologist for thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately as early as 11 days post-conception).
If pregnancy gets confirmed, consider requesting your gynecologist/obstetrician for getting serial HCG levels tested. Doubling up of the hormone levels in 48-72 hours will show that pregnancy is going on smoothly. If levels are not adequate, then external hormonal supplements should be considered to be started to help with preventing miscarriage/threatened abortion. Take adequate rest. Drink plenty of water. Maintain healthy diet. Avoid any heavy work. Avoid having intercourse during first trimester. Avoid lifting heavy objects and climbing stairs or standing for long periods of time. Report any unusual symptoms immediately to your treating doctor/gynecologist. Be in regular monitoring and follow-up with your treating doctor/gynecologist.
Take care.