What to do if I don't recall when I took my last tetanus shot. Should I be concerned?

Yes. Standard risk people should have a tetanus shot every 10 years. High risk people should have a tetanus shot every five years.
Depends. If you can't remember it is likely 5 years since your last shot. If activities like gardening expose you to spores you should consider a booster. Diptheria deserves an update too. If your life ever brings you in contact with infants, you should consider the tdap which adds coverage for pertussis, which adults can get even if immunized as babies. This would prevent you from passing it to a baby.