You should not keep the seatbelt loose - in the event of an accident the seat belt is what is keeping you safe... too loose and you can get significantly more serious injuries, nearly as bad as not wearing it at all.
Your baby is fine.
You and baby should both be fine.
I think you'll be fine. Sounds like your seat belt just locked up. Your baby is pretty safe in there, protected by a lot of fluid. I'd just let your OB or Midwife know at your next appointment.