I ain't never tried to grow tomatoes in pots but I do know they have large root systems. This year I started late and they tomato plants I got were pretty sickly looking. I just made my rows then put miracle grow potting soil down the middle and planted my stuff. They looked bad for a month or so then perked up and everything in the garden is outstanding. I'm gonna guess that yer pot is a bit small, but it could be the nitrogen levels too as one post stated.
Good luck with your plants,
Gordon

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better have that checked