If the leaves are yellowing, it might be due to excessive watering. Add a small amount of a balnced fertilizer, not a harsh chemeical based one, but something organic. You don't really want the plant to grow too big anyway, as once it shoots up a stalk of thin leaves it is now in 'bolting mode', and cannot recover back to noraml leaves anymore. Cilantro has soft weak stems which is quite normal, after it starts to bolt the stems thicken and get fiberous and woody. They do like sun, but not a lot of heat. If the pot is small, the roots might be too crowded.

All of the points that Ksrogers made are good but the bottom line is that cilantro, like dill and a few other fast growing annuals are poor choices for potted plants.

I suspect light is your main problem, but even with adequate light the useful life of the plant is limited.

I'm on my third planting of cilantro for the season. I sow thick, water well and the cut it for bunches. It will regrow but the quality is never as good.

You shouldn't hesitate to reseed and possibly the second seeding will outperform your present potted crop. From seeding til 2" stage should only take 3 weeks with adequate light and moisture. My suggestion would be to plant thick (50-60 seeds in a 6" pot) so you can harvest plenty over a few weeks time.