Haven't brushed teeth in a while and when i did, i feel a small sore in my mouth. is this due to lack of oral hygiene? how to fix this and prevent infection?

The sore is probably a recurrent aphthous ulcer which should resolve on its own within two weeks. If the lesion does not resolve have your dentist take a look. And start brushing your teeth 2x a day, floss, and see your dentist twice a year for exams and professional cleanings. You would be surprised how quickly your teeth will deteriorate without proper home and professional care.

It is probably a canker sore if it is painful from a buildup of acid and poor oral hygiene. Start brushing and flossing and have a diet that is low carbs and acidic. Best see a dentist for checkup.