Hi I feel the blisters you mean is wounds (ulcers) on your tongue. By spreading you mean it is appearing in fresh areas or the wound is increasing in size? There can be many conditions causing ulcers on tongue like viral infection, anaemia, vitamin deficiencies etc. To arrive at a proper diagnosis more details are needed like your sex, teeth status, if you are on any longterm medication, if you have any eye or joint problems etc. Stress can definitely precipitate this. To manage stress do meditation daily, take regular exercise like brisk walking for half an hour a day even though you are busy preparing for exams. Have nutritious food and enough sleep (at least 7 hours a day) You can take iron and B complex supplements. Have a dental check up. Even with these if your problem persists show to a good ENT specialist.