The first headache surfaced two weeks ago and Mariano Rivera described it as more than the garden variety discomfort that two aspirin could make disappear. “It was severe,’’ Rivera told The Post yesterday morning in a George M. Steinbrenner Field dugout. The headaches weren’t as strong, but they didn’t go away. So Friday the Yankees sent baseball’s all-time saves leader for tests to determine the cause of the headaches. “When you have issues you want to make sure everything is OK,’’ Rivera said of the CAT-scan and blood test. “They said it seems like migraine headaches.’’ Rivera said doctors had not yet told him how the headaches will be treated. Yesterday, Rivera said he felt fine and that he would pitch an inning today against the Rays at Steinbrenner Field. According to GM Brian Cashman, the Yankees weren’t frightened by the situation. “There are protocols and we ran through them,’’ Cashman said about sending Rivera for tests. “Even if there is one-half of a percent [of a problem], you want to rule it out. We weren’t worried about it.’’ Rivera said he had never experienced migraine headaches, but one of his sisters has had them. Cashman may not have been overly concerned the tests would have discovered something serious, but lately tests on Yankees players haven’t produced very good results.