Having had a car accident can be very stressful, and stress is a known cause for hbp.
Pain from an injury causes a surge of adrenaline and a rise in heart rate and blood pressure. When your adrenaline levels fall back to normal, your blood pressure and heart rate do as well.
As Jon states some medication (usually anti-inflammation) can make your blood pressure go up by affecting your kidney's ability to get rid of water and salt. So, people using these medications need to have their blood pressure checked periodically. If one has a heart condition, the added volume of blood from the medication can add to heart problems.
Pain can certainly cause your blood pressure to be elevated. However, so can some of the meds given for pain relief. If you were prescribed any of the non steroidal anti-inflammatory drugs (NSAIDS) which include ibuprofen, these can also increase your blood pressure. You may want to look up the side effects of any prescriptions you were given or ask your doctor,
Hope this helps,
Jon