Where is moisture in my attic coming from?

I had a mildew issue in my attic which was remediated (sprayed, scrubbed, painted with some protectant paint, and dried out with a space heater). We didn't have any issues with mold growth in the the house but we obviously didnt want to leave it unchecked, and no problems ever since. This winter I poked my head into the attic and noticed a good amount of frost, not a real thick coating, but a decent coating on the flat boards and nails. Ive been trying to figure out where the moisture is coming from. Nothing leaks when it rains, I sealed up every type of spot where heat could be getting up there through ceiling fans and things like that. Recently during a rain storm I noticed some water looked as though is was going behind my gutters and dripping off from underneath the gutter. Out of curiousity I looked back up in the attic, and sure enough there was a small spot of mildew in the proximity of where the water was running behing the gutter outside. What are the chances that water getting behind the gutter and soaking the fascia board could be the cause of the moisture in the attic, and if so is there a simple way to fix this? Thanks for any help, its would really set my mind at ease. Im going to take some pictures tommorow of outside and in so if those would help