My tomato plants have developed yellow spots on the leaves that are spreading quickly
That sounds rough your tomato leaves turning yellow and spreading fast could just be a stress or a disease on the plant Lower leaves often show problems first.
In my garden early blight showed up as yellow spots that spread I pruned infected leaves and boosted airflow which slowed the spread.
Not sure what it is could be a magnesium deficiency or a disease the quick spread makes me lean toward disease but more checks would help
Best next moves water at soil level prune infected leaves mulch to keep leaves dry and consider a labeled fungicide if many plants are affected