i planted Coreopsis last year and it did great. but now in that same spot I have this plant. what is it? is it coreopsis? it doesn't really look like it.

looks like - four o'clocks

Four o'Clocks (Mirabilis jalapa)

Four o'clocks was my first thought too.

ugh, i think you're right. i did have 4 o clocks growing in that area last year. i guess they spread their seeds. but what about my poor coreopsis? was it overrun?

I don't think four o'clocks would overrun coreopsis. perhaps it just didn't survive the winter or your spot might of been less then ideal.
hard to say , mine were pretty much carefree in partial shade but outgrew their spot, I divided them and moved them to full sun last year and had to water them a little more often but both small clumps did survive and are happily blooming again.

There are perennial Coreopsis, and annuals. Which kind did you have?