It sounds like you had some trismus and possibility some cellulitis. I am guessing that there was an infection that was pretty hard and swollen and it may be pinching/pressing on your muscles that open your jaw and its making it harder to open. As the infection resolves from your own immune system and antibiotics, your swelling will go down and it should be easier to open your jaw. If it doesn't improve in about 3-4 days after starting antibiotics, I would contact your dentist to see if there may something else such as a muscle strain/pull.