Last 3 days i have a dull pain/discomfort just below chin(between mental tubercle and left anterior digastric muscle).it's only when i chew or shallow,not all day,only sometimes.had been to dentist 5 days ago and had mouth open very wide.what can be?

What was the reason for the dental examination? Have you had an active infection in your oral cavity? Open mouth too wide in a prolonged period may cause a muscle strain which may cause discomfort with chewing but not swallowing. However, whatever the cause is, those symptoms should be self limited and you should be fine within the next 3 days. Take care.

Post-surgical soreness. Dental procedures are for the most part surgical. you now have sore soft tissues. Speak to your Dentist about what to do to mediate the soreness.