It might be time to see an allergist. I would get her tested ASAP to find out if this IS allergies (it might not be - so rule out allergies first) and if so, what exactly she's allergic to. Just because she's on allergy meds does not mean they are the right meds targeting the right allergens.
If she's been diagnosed by the pediatrician with having "seasonal allergies" (namely, seasonal plant pollens), I'd get her to an allergist for a more complete and detailed diagnosis, since clearly the current allergy meds aren't working.
If it turns out she has no allergies beyond what the current meds affect, then at least you have ruled out allergies. If that's the case, it would be time to move on quickly to another specialist. I would consider a specialist who focuses on headaches -- does she have migraine-type headaches? Do things like light or noise set them off? A pediatric neurologist could be the next stop.
I say this on questions here a lot: Be assertive with her pediatrician and just insist on a referral ASAP to an allergist, and then if needed to another specialist. You have to be quite assertive with many doctors who may dismiss things as just "a kid who thinks she's having headaches" or "it's just allergies." You know your child best. If you think this is really something, then don't ASK for a referral, TELL the doctor: "It's time for us to see a specialist. I am asking for a referral by the end of the day for an allergist so we can get an appointment ASAP." Some doctors are very hesitant to refer patients to specialists because the doctors think it makes them look bad, or the doctors get flak from insurance firms about doing referrals, or both. Don't let that get in your way. Just say, you want a referral now.