Could I be pregnant and have negative test? How would I know that I should see a doctor? I have pregnancy symptoms.

Home pregnancy tests. Home tests are fairly accurate though if left too long can give you a false reading. Stick to the directions. You can often get low cost pregnancy testing at planned parenthood or many local crisis pregnancy centers (sometimes free).
Pregnant? Home pregnancy tests are fairly reliable. You can repeat the home pregnancy test but betteri to see your doctor to repeat the test to be sure.