First, if you just got it in April, I'm not surprised it didn't grow much. Rule of thumb for perennials is this: the first year they sleep, the second year they creep, the third year they leap...give it some time to get established. Second, I'm not sure I'd try to grow an echinacea in a pot. If the pot's not big enough, there won't be enough room for its roots to spread out. Yellowing leaves usually means the roots aren't getting enough air - it's common to see it with plants that are getting too much water - the roots don't get enough of a chance to breathe and the leaves turn yellow, which would make sense if yours is in a pot that isn't allowing the roots enough space. And, I'm not sure what zone you're in - if "nyc" means New York City, then you've got a cold winter ahead of you and those roots are going to need more insulation than they will get in that pot (and it won't do well if you try to bring it inside). I would pull it out of the pot and see if the roots are indeed filling up the pot. Watch how much you water it. And if it's at all possible, find a sunny spot and plant it in the ground. At a minimum, get it into a much bigger pot and wrap a blanket around it for the winter. These are native plants, they grow in almost any kind of soil, they're pretty hard to kill, and they're not at all fussy. They do have the same basic needs that all plants have, though - enough, but not too much moisture, and room to spread their roots.

Sometimes things just die with no explanation. Sad. ;-(

we've had the wettest spring and summer in decades in central north carolina, and a couple of my cones, as well as a bunch of rudbeckias, have quickly browned, shriveled, and died away. I've pulled a couple of affected plants and noticed the roots were mushy and stunted. One of my garden books lists both echinacea and rudbeckia as plants that are sensitive to fungal root rot in areas of high moisture and poor drainage (which is what I've got this year). Same thing has happened to the thyme in my herb garden. In general, the rot seems very hit-or-miss -- I have several of each of these plants that are doing just fine.

Echinacea have a reputation for doing that when they are receiving too much water. And it only seems to affect the terminal, budded ends. We used to see it all the time at the nursery when the seasonal staff got over indulgent with the watering wands :-) Make sure they are planted in very well drained soil and maybe cut back on any irrigation.