A fever following cold or flu is not always pneumonia. Well pneumonia can occur like in that fashion but it is not always a rule. As of now you can watch for some warning signs for pneumonia / lower respiratory tract infections :
1. Progressing and persisting fever for 24 hours or more from now
2. Increased breathing rate - persistently more than 40 breaths per minute
3. Severe chest indrawing, troubled breathing
If any of these symptoms are there get medical help as early as possible. If there is only mild fever responding to medications and if it is not persisting then there is nothing much to worry. The fever, in that case is mostly part of the flu.
Hope this helps
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