Could you be low on Zinc? Mouth ulcers are a sign of the body being 'down' ...... Your GP should at least take a throat swab to see if anything is 'growing' which would aggravate the ulcers.
If your GP ain't helpful, mayB ring your Dentist? Boiled, salted water cooled is what mine suggests following dental work, swilled round, held then spat out.
How long have you been swallowing Lanspronal, could it be the coating that is aggravating the tender mouth lining?