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While laying down human respiratory capacity decreases. This is due to several factors like pushing up of diaphragm, apparent decrease in lung capacity etc. The flue could have already compromised your respiratory functional capacity, which was already compromised.
There is nothing to be worry, but need to be careful and watch for any danger signs. The danger signs would be fever, sudden onset of breathlessness, progressive breathlessness , breathlessness while lying only on side. sever cough, leg swelling and decreased urine output.
As the per natural course of the illness (viral infection . flu) you should see resolution and improvement in symptoms in less than 3-4 days. If that is not the case you might require a chest xay and antibiotics.
Hope this helps'
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