i would think that ur allergies is getting worst...i am not a doctor so that is the best thing that i could come up with
Do you remember burning the top of your mouth? Sometimes it can be due to trauma to your mouth, like if you were eating a snickers bar and the peanuts scraped the top of your mouth. (Happens to me). Sometimes your toothpaste can cause the tissue to slough off. There are some things you can use for the dry mouth, since it sounds like you are a mouth breather. Biotene is found at your local drug store, and they have several products you can use, spray to lubricate your mouth, mouth wash and tooth paste. If the ulcerations last longer than a few days, I would see a dentist. If it is uncomfortable, you can make a mouth rinse using Keopectate and liquid Benadryl. 1/2 tsp of each, mix together and swish for one minute, then spit out. It will help coat the lesion and numb it at the same time to give you relief. Good Luck
do you use any denture ? this may be the cause if so please check your dentist,
other may be fungal infection that give you this ulcer in the palatal area and cheecks a dentis can diagnose
third , this may be due to blood disease so you need to see a haematologist for this disoder