I had one nijure back left side of my head before...

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could head injures cause damage to brain :

yeah,  it could effect your inner brain and could cause damage such as brain concussion or internal bleeding or hemmorrhage , but usually effects appear quite directly and soon , and usually patient will go through different medical examnations to determine if there is a brain damage during head injury 
- in your case : its unlikely to have brain damage due to late symptoms  you are mentioning which is one month , and we are guessing that you have already get into medical examnations or your injury was simple and you have no brain injury signs .

what is the head injury serious signs :

1. loss of consciousness 
2. altered mental status 
3. vomiting 
4. external bleeding 
5. blurred vision 
6. slurred speech or effect speech 
7. external swelling 

what are the possible causes of your medical condition :

1. external damage to the outer layers of your skull 
2. some types of headache 
3. new injury 

what to do next :

1. visit your internal medicine doctor and explain your medical condition 
2. have a plenty of rest , avoid smoking and high brightness screens 
3. apply anti inflammation and pain killer ( topical ) on the pain area if there is an external swelling 
4. doing the tests and needed medical examnations to roll out any of serious illnesses 
5. follow your doctor instruction and take good care of your self 

needed medical examnations :

1. clinical check by doctor 
2. blood test 
3. vitals check 
4. skull X-ray
5. ct scan for skull 
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