What to do if i have got an ulcer above my toothache, is it serious?

Maybe not----but: An ulcer above a tooth could be a draining abscess , which could get serious. It may be an aphthous or herpetic ulcer. Sounds like it is time for an exam.

Probably an abscess: Sounds like the source of your pain very well could be an infection in the tooth that has spread to involve the supporting bone. These are usually slowly progressing problems, but can become serious quite rapidly under the right conditions. You would be wise to seek evaluation by an endodontist or general dentist for treatment.

Could be: A toothache can be caused by a non-vital nerve or a deep cavity... If the former, it may very well explain the 'ulcer'. Then again, you may have a canker sore in that area which is manifesting itself as a toothache. A visit to your dentist will give you the proper diagnosis.