I recently purchased a patio lemon tree at a lawn and garden center. The tree flowered and produced one lemon. It has continued to flower, but recently in the past couple weeks, all the leaves turned yellow and fell off within a couple weeks. The flowers are still there and blooming. Any indications of what this means? I recently repotted it (once I purchased it 6 months ago) so it has fresher soil. I purchased a fertilizer that was specially made for trees too and this seemed to happen a couple weeks after I first fertilized it.