Is my cycle changing?

Hey, everyone! I've had my period since I was 10 years old, and I'm currently 17. I've never missed a month, and my periods were normally around 28 days per cycle. But all of a sudden, it's been changing to around 40 days, as shown in the picture. I'm not on birth control, but I've had protected sex before. My period is supposed to be on May 15th, and I'm experiencing some kind of pale brown discharge, and I never have discharge like that until my period is about to start. But I know it will be near whenever eve says. I'm kind of concerned as to what could be causing the sudden changes, and what I could do? Thanks!! 💕