This topic is where personal preference really plays a role. Location and weather conditions also play a big factor. There are a lot of different types of "raining". Since you don't have a ripstop poncho, then my guess is that it is vinyl, or PVC. Regardless of the material, it sounds like you have a non-breathable poncho. Since it is non-breathable, it is highly likely that you got soaked from within. Your own sweat couldn't escape and pooled around your neck and shoulders.
Try this, put on a dry cotton t-shirt and then put on your poncho. Go stand in the shower (with it on, of course). Make sure water is flowing over your shoulder seam. When you get out of the shower, check your t-shirt and see where it got wet. If your shirt is wet, then that is where it is leaking. If it isn't wet, then you were soaked by perspiration.