Suggest Treatment For Vertigo, Pain And Stiffness In Body
You should be fine.
Detailed Answer:
I read your question carefully and I understand your concern which i justified since coming off a recent concussions can render one more vulnerable to damage from trauma in the following first weeks.
However in this case you state not to have hit your head and it was a hit from a car seat which has some cushioning covering it. Also the force of impact as abrupt as it might have seemed to you caught by XXXXXXX still can't have the same impact as say a motor vehicle accident or a fall for example.
As for the stiffening it may be partially due to the stress and fear increasing muscle tension temporarily apart from the therapy.
So in conclusions I do not think you should worry much about it, you should be fine.
I remain at your disposal for other questions.
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Detailed Answer:
I agree with your doctor. It certainly can't cause a concussion, don't think it will do much harm to the arm either.
Let me know if I can further assist you.