I have had my tongue pierced last saturday and there is a small mouth ulcer under my tongue. what should i do?

Rinse: In the absence of swelling, the thing to do is rinse 4 - 5 times a day with warm salt water, bathing the area, for 3 - 5 days. Avoid spicy foods. If it does not improve in the next 3 days, see your dentist to check it. It is likely an aphthous ulcer which takes 10 - 14 days to heal. But, since you had trauma in the area it could be something else. Keep an eye on it.

Remove piercing: The tongue is in constant movement from speaking, swallowing, eating, etc. The metal in your tongue is abrading the floor of your mouth, thus the ulcer. Either remove the metal alltogether, or remove it and wait until the ulcer heals, then place jewelry with more rounded surfaces.