In general your teeth should not be shifting that quickly. The speed of teeth shifting / relapsing back into old position depends largely on the angulation of the tooth roots and the final bite position of your teeth. Taking an xray helps to check this out.
In short, it is unusual for teeth to shift in a week so dont worry too much about it unless you personally see significant shifting occuring. A suggestion would be to call your dental clinic to ask if you can get the retainer earlier though at times there may be an express service fee charge for expedited cases.
Hope this helps!
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