This is fairly common with pollen allergies; rain/humidity can take much of the pollen out of the air. It also reinforces the suspicion of something airborne. Changing weather also involves pressure changes which can affect sinuses (as glenntaj mentioned), but they occur just prior to weather changing.

Allergies can come & go throughout our lifetimes, so you may have more than just birch pollen.

Your age pretty much eliminates any middle-age phenomena , however problems concentrating and forgetfulness go hand in hand with adolescence. Adolescent—or teenage—brain fog has been written about, and is nothing to be too concerned about—you'll outgrow it.

You realize we're not doctors here, and we're just spitballing. Whatever it is, don't let anxiety/fear over this consume you—being 15 is stressful enough, and most of what we stress over are things we all go through in one way or another.

It also couldn't hurt to get checked over by your family doctor.

Doc