Thanks for writing to the forum!
This could be due to GERD or acid reflux to your throat and mouth. These could also be canker sores. Chances of them being canker sores are high. You can try some ointment containing benzocaine to help relieve pain. You can clean them with calamine lotion or with saline water by using these to rinse your mouth. Take some multivitamin tablet too.
Fungal infection too can cause white spots as do bad oral hygiene. Smokers should be careful about such lesions as they can be precursors of oral cancer.
It is difficult to comment beyond this at this stage. Do consult your family physician if these home remedies do not help. Please let me know if there is any thing else and do keep me posted. Take care!