Hi Celeste, thanks for the question.  

This looks like septoria leaf spot.  A rather common disease for tomatoes.  It can infect through water splash, wind, tools (including hands, clothing, etc.), and infected debris.  Nearby weeds can harbor the disease.  Treating it requires removal of infected leaves.  A fungicide (copper will help with bacterial issues also) will help prevent infections especially if applied before infections occur. Sanitation can really help. Avoid overhead watering. Check out the following links:

Vegetable: Solanaceous, Septoria Leaf Spot | Center for Agriculture, Food, and the Environment at UMass Amherst

Tomato Septoria Leaf Spot – UW Vegetable Pathology – UW–Madison (wisc.edu)

Tomato leaf spot diseases | UMN Extension

I hope this is helpful.