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You did not mention your state, so we do not know which state laws to apply to your question. Do you live in a "no-fault" state?
Report this to your auto insurance company, and find out if you have either med-pay coverage or PIP benefits on your auto policy.
Most likely you have sustained a sprain/strain type injury. This can be painful, but the good news is that you can expect a full recovery, with or without treatment, in a matter of weeks. Usually 2 to 6 weeks. Getting medical treatment will not make your body heal any faster, but a doctor would be able to rule out any additional injuries.
You may have a claim against the party that rear-ended you, but that will depend on what state you live in. If in a No-Fault state, probably not.
You do not need an attorney. There is no dispute pending. You can retain an attorney later if necessarily, but most likely you will not need one. And attorneys do not work for free. They will take part of the settlement money.