While I wouldn't panic, I would call again. I agree, it will probably get solved as aerobod describes, but I would also ensure that your 'special needs' request is on your PNR.

I arrived at LAS two hours early on crutches and had the pleasure of sitting in 19D (yes, it was an aisle, but getting there was not a treat). The flight crew were very apologetic, but being a full flight, at boarding time there wasn't anything they could do. They were puzzled as to why it wasn't taken care of during allocations, or at least at checkin or the gate, as I had requested a wheelchair ahead of time.

Hope it all works out!