August mri with contrast showed brain lesion they say was very small and nothing to be concerned about. could it have turned into a malignant tumor ?

Unlikely.: These days people who read MRIs and other studies do a very good job of making a plan for follow-up that will minimize the chance of missing something dangerous. If they were not worried, then you should not be worried. Just follow their recommendations for follow-up testing if needed and you should be fine.

MRI incidentaloma: MRI is very good at seeing things in the brain even very small and inconsequential thingsI think it would be extremely unlikely that your doc would tell you it was nothing to be worried about AND THEN it became malignant. However, none of us have see the report directly so it is hard to comment. You will have to ask your doc specifically.

Can't say: Can't say without seeing your MRI, but if you are not having symptoms and the lesion appears benign, then I would not worry if your doctor is not worried. A good question to ask your doctor is, "What would you do if you were in my situation and this was your MRI?" If the answer is nothing, then I would be pretty reassured. You could also ask if a repeat MRI would be wise to check for changes.