Given the weather over the last week, you may have already found the answer to your question. Guavas thrive in places like Fiji and barely grow in the top of the North.
So it sounds to me like your plant may well be suffering from frost burn. Covering it with a frost cloth, available from garden centres, could help it survive the winter. If you haven’t had a frost yet, the cause may be an insect like thrips. The best way to beat them is with neem oil.