Why does coveralls coverage keeps decreasing when I add more unit tests?

I am writing unit tests for this C++ project using Google Test framework. I have a weird problem wherein, when I create only one test suite, Coveralls gives a code coverage of about ~70 %. Whereas when I add two more additional test suites (extra ones to the previous) the code coverage drops very steeply to about ~25 %. I am very confused why does this happen ? Is this normal ?