We had the exact same situation with our son this year. He'll be 3 in December and starting in August until about 1 week ago he was constantly sick. He had bouts before of sinus infections maybe 2-3 times a year, but this time it was chronic and we couldn't get it cleared up.
Here's what we did-which sounds similiar to you...Our ped referred us to an allergist. We know him since he is my husband's as well so were confident he'd be helpful. He did a scratch test rather then blood test to determine allergies and our son is allergic to A LOT. Especially seasonal which explains why he started getting sick in August thru early October, (we took him to a Pumpkin Farm and we were hardly out of the car and he was congested).
He prescribed Zycal with is a stronger version of OTC zyrtec and a nose spray. It is helping, but I can tell it's the weather changing that is really doing it-that and clearing the sinus infection for good. BUT I would say this for you-if your daughter has had a 'cold' longer then 7 days she probably has a sinus infection. That's what both doctors told us. Plus they feel crappy and are crabby and all that.
I would check with another ped. If it's been this long she more then likely needs an antib. and probably a long round of it. We go through this constantly with my husband and not probably my son as well. Don't be araid that she'll be on one 'too' long. Our son had to go through 5 rounds this year - 4 different drugs-before they found one that did the trick. And yea-I'm not a fan, but I look at this year as the trial year for what works and what doesn't and hopefully from now on, now that we know his allergies we can get in front of this next year.
Also-get a probiotic, (whole foods) for toddlers to help counter act the antib' in her system.
Good luck! Once our son was feeling better it was like we got our kid back!