There are a few reasons you could be getting wet inside your rain jacket. The most common reason is that there is a fail point on your rain jacket, typically along the zippers or seams, where the waterproof membrane is failing and rain is soaking through. It’s also possible, especially if you’ve been in a serious downpour for a while, that the waterproof membrane has simply hit its saturation point, something that can happen with both breathable rain jackets and some lightweight rain jackets. The final reason you might be wet inside your rain jacket is that your body’s sweat is trapped, and unable to evaporate to the outside air.