Have the plants been growing outdoors for a week or more? It looks like I see a bit of purple on the foliage and that is the sign of cold weather damage. Even if the days are warm it is the night temps that will stunt young tomato plants. They like the nights to be above 50degrees and in Western Washington that usually means waiting until June to put the plants our overnight. Gardeners with a protected spot close to the house and with raised beds may be able to jump the gun a bit. Too much water could also be damaging the plants so do not water until the top one inch of soil is dry. Soil that is too damp can cause mildew and do I see some white powder on the leaves? There is still lots of time to start with new plants if your tomato plants don't perk up soon.  Keep growing, Marianne Binetti