There are a couple of different explanations for this:
First, if you're using a different type or brand of test each time (say, a First Response Early Response pink line test followed by a ClearBlue digital) then keep in mind that different tests test for different levels of pregnancy hormone. This means that you could have gotten a positive the first time because the test you used tested for a smaller amount of pregnancy hormone than the second test did. It doesn't mean you're not still pregnant, it means that the hormone needs to build more to get an accurate result with a test that requires more pregnancy hormone.
Another possibility is that the implantation bleeding wasn't implantation bleeding, but an early miscarriage. The first test may have showed positive because there was still enough hormone in your body to give you a positive test result but as the miscarriage progressed your body had less hormone, giving the next test a negative result.
If you're unsure, the best thing to do is wait for 4 days after your suspected implantation bleeding, then test with a pink dye test for the most accurate early testing result.