Certified GKH Gardening Expert
Depending on what you fertilized, or did not fertilize with - it could be nutrient deficiencies. Usually this can be caused by a lack of phosphorus and potassium in the soil. Sometimes this can be indicative of too much nitrogen in the soil as well.
If you are sure that there are plenty of nutrients in the soil, then this can have other causes as well. A pH swing, downward, can cause these symptoms. A soil test will confirm. An EC (electro-conductivity) meter can confirm nutrient content but not exactly how much of what is present.
Your local extension service can test your soil, or you can test it yourself.
These articles will help: https://www.gardeningknowhow.com/extension-search