I had a very light positive hpt went to the doctors and they said urine came back negative. still have brown spotting. can implantation last a month?

Too early: You should repeat the home test in one week. If there's a pregnancy that's viable, it should be positive.

No: If you had sex since your last period and missed your period by 4-5 days, do a home pregnancy test. If the result is negative repeat the test 4-5 days later unless you get a period by then. Use first morning urine and follow the instructions for the test carefully.