Have swollen lymph under jaw for a month with on and off ulcer in mouth......causes and is it serious?had protected intercourse last year late may

Aphthous ulcer : It sounds like an aphthous ulcer. They are usually no bigger than a pencil eraser, are painful and can cause a local node to enlarge. In some people they may be recurrent over time without having serious cause. Multiple ulcers at once, especially with other symptoms or one ulcer that persists more than two weeks needs more investigation.