Why are my tomato plant leaves turning yellowish with black spots?

sounds like fungal disease Fungicides can help if applied early enough and thoroughly. They won’t kill the fungus but they will prevent the spreading of the disease which is certain to occur. Be sure to follow product instructions. The best control for all fungal disease is good cultural practice.

Here are some very important cultural practices to reduce tomato disease in your garden:

1. Remove old plant debris. Many common weeds can host fungal disease. Do not compost this material or any plants that have any signs of disease unless you are completely confident that your pile gets hot enough to kill the spores. During the growing season spores from infected plants can splash onto you new tomato plants and the disease cycle will continue.
2. At the end of the growing season, all tomato refuse should be discarded.
3. Rotate your tomatoes to a different location each year. Remember that nightshades (tomatoes, eggplant, peppers) and potatoes all have similar disease problems so rotate all to new locations. A minimum of 3 years is recommended to keep the soil-born fungi at bay.
4. Alter the microclimate. Grow in full sun. Stake or cage to keep leaves off the ground. Remove any leaves that touch the ground and as you harvest your tomatoes, trim the leaves from the ground up. Fungi thrive in damp conditions so get as much air circulation as possible particularly if in a humid climates.
5. Water only in the morning and try not to get the leaves on the plants wet. Don’t let water from the soil splash onto the plants. Soaker hoses are particularly desirable for tomatoes.
6. Mulch thickly (6 inches) but pull mulch slightly away from the tomato stems. In addition to preventing soil-borne disease, the thick mulch will also discourage weeds and keep the soil moist.
7. Monitor and control pests on plants because sucking or chewing inscts provide open wounds which invite fungal disease.
8. Plant on well drained soil. Fungal disease thrives on moisture so good drainage is critical.
9. Avoid high nitrogen fertilizers. They cause the plant to have many leaves which reduces air circulation. Tomatoes prefer calcium and phosphorus fertilizers.
10. Plant at the right time of the year for your area. Tomatoes grow best when daytime temperatures are 70-80 degrees and nighttime temperatures are 60-70 degrees. So Southern tomato growers are planting in March and far North growers may be planting in May.
11. Clean garden tools after use. Just one swipe with an infected tool can spread fungal disease. After removing dirt, soak your tools for at least a minute in a disinfectant solution. 1-5 chlorine bleach was always the standard recommendation although prolonged use could pit or mar your tools. Disposable bleach free disinfectant wipes are hand for smaller tools. Other options include Lysol, Pinesol, rubbing alcohol or hydrogen peroxide.
12. Buy tomato plants that are disease resistant.

Hello Meda, check out this link, it sounds like you have septoria, the link will provide the info you need to lessen the effect,

https://www.canr.msu.edu/news/septoria_leaf_spot_on_tomatoes_preventing_spots_before_your_eyes

That's a fungal disease. It's very common in warm, humid climates, like my state of Virginia. I would move my plants to another bed next year as the fungal spores can live in the soil and splash up onto new leaves in the following years. Yes, over watering can cause it, but so can just plain old rain and humidity. Here's more info to help: https://njaes.rutgers.edu/fs547/