What Causes A Delayed Period?

Hey, I was dignosed with a kidney infection a couple days ago, but I caught it early. I had one before, but I waited a long time before I went to the doctor and it really didn't look good. But this time I recongized the symptoms right away. However, I am sexually active and I do not use protection. I am a bit irregular, but I'm always early, I've never been late. Right now it's only two days, but I am still a bit worried. Could it be from the infection, medication, or should I take a pregnancy test soon?