It is possible for some plants to send out a "sport" which is a genetically altered shoot. This is how many new varieties are found. However I am not sure about your plant. If you did not see any white blooms earlier it may have been a lavendar variety all along.
The leaf spots sound like a fungus. Products with chlorothalonil (Daconil is one brand) are usually pretty effective. Your best bet is to take some leaves showing the spots to your County Extension Office and see if they can identify the disease. They would then be able to recommend an appropriate, labeled product for you to use.
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