Answer: If the yellowing leaves are at the bottom of the plant - the oldest leaves - then it could be normal. Plants do lose their oldest leaves, especially when they are busy producing healthy new leaves to the top of the plant. If the yellowing includes the newest plus the oldest leaves, it could be lack of nutrition, or over-watering. There are two diseases common to tomatoes, so if you don't think the above applies, it could be a disease.
Irregular brown spots with a "bullseye" pattern on lower leaves of plants. Spots enlarge to 4-9 mm (0.25 - 0.5"), then coalesce. Affected leaves turn brown, drop. Stem spotting and stem girdling or fruit rot (near stem) may occur late in season. Rotation and good sanitation practices (including removing horsenettle, jimsonweed, groundcherry and nightshade) are recommended controls. Also Bordeaux mixture.
Fusarium wilt: ("yellows")
One of the most common and damaging diseases. Generally not damaging unless soil and air temperatures are high in much of the season. Slight yellowing of a single leaf, or slight wilting of the lower leaves is followed by an overall yellowing. Brown discoloration of vascular tissues in stem or petioles of wilted leaves is common. Choose Fusarium resistant cultivars, grow in clean soil, rotate crops, destroy vines at end of the summer. Nematode infection increases risk of fusarium infection.
Hope this information helps you determine what the problem might be.
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