Go to planned parenthood, they will test you for pregnancy. If your last period was February 18th, you should have a bright blue test (aka LOTS of pregnancy hormones). It shouldn't be 'iffy' whether or not the test is positive.
In addition, if you read the test after 10 minutes, an evaporation line can be present which can make tests appear to read a faint positive when that isn't what is going on at all.
Re-take the test with your first morning urine as the directions state. Read the test within the time frame it says on the box. If you do not trust the test, go see a doctor or your local planned parenthood office.
Edited to add: I AM SO SORRY! I read a title for LMP Feb 18th and then thought that was you. Sorry.