Something that happened earlier (Trigger Warning) (POCD)

This morning I woke up pretty early before my alarm even went off. A lot of times when this happens, I'm in a half-awake and half-dream state with a lot of the weird dream images (I'm guessing) going through my head. One of these was some vague image of someone young. I don't remember the details, but I know there was no sexual content. Anyway, I felt a little weird down there when this was going through my head, checked myself, and felt a partial erection. This bothered me a lot, but I was so tired that I fell asleep until my alarm went off later and I woke up. Well, later on I decided to test myself. Normally I've been good about not testing myself through thoughts (I usually examine my reactions to things I see) because it bothers me so much. There was no sexual content in the thoughts. I just imagined images of children to see if I got aroused. Fortunately, there was absolutely no erection when thinking about this. Then I started fantasizing about things I enjoy and got a partial erection (but not like the one earlier). I know I shouldn't worry as much about this. I'm only 19 and these random erections are common, but I don't know. On top of POCD, when I have reactions to things I feel I shouldn't, I feel guilty and tempted to confess it. I know I shouldn't feel guilty about thoughts and feelings, but it's really hard.