My left tonsil had a canker sore on it but now my right tonsil has a canker sore on it, the sores hasn't got a border around it, what could cause it?

Be sure of diagnosis: Get examined please. Canker sores are common aphthous ulcers of mouth that occur situationally, often in response to stressors, are bacterial or viral, & can be treated topically with many otc products that reduce local discomfort & potentially speed healing. However, oral/dental problems & a number of systemic diseases cause canker sore-like lesions--crohn's, behcet's, STD to mention a few.