I always get pimple under my tongue, what can be the cause of it? Usually one at a time, approx 1-2mm in diameter, afrer days it turns into an ulcer.

Mucocele. Could be a mucocele. Underneath the tongue is a very common place for them and it may be rupturing after a few days looking like an ulcer. Mucoceles are caused by an obstructed or ruptured salivary gland duct, hence clear fluid within. Some mucoceles spontaneously resolve on their own after a short time. Others are chronic and requmucire removal through minimally invasive sx or the use of a laser.
Pimple under tongue. These bumps are usually related to salivary glands either a blocked duct that swells called a mucocele or a ruptured duct called a ranula. Cause is not clear, but the ulceration at the end is not unusual as this is traumatic to the tissue, albeit minor in most cases. If it continues happening it could be related to diet or meds side effects. You may want to consult your dr.