When you use two different brand of tests you run this risk because each test has its own sensitivity. Sounds like you used a really sensitive test the first time around and a not-so-sensitive one the second time.
Also, you'll get a more accurate result closer to Friday. So definitely re-test! You may be pregnant.
i'd personally wait till Friday to test. First one could have been accurate, and maybe the second one just doesn't pick up low levels. So test again on Friday! Good Luck!