You are correct about Ant-Man it is post Avengers 2.
Allegedly, the Incredible Hulk movie is set before Iron Man.
They COULD (not that I think they would) do something similar with Avengers 2/Ant Man. It'd be kinda ironic if, after the credits we hear something like:
"Whatcha got there, Pym?"
"Check it out, Tony, I've designed a new AI! It's like JARVIS but capable of even higher reasoning. I'm calling it ULTRON and it's going to change the world!"
*Cue audience reaction, having seen Ultron attacking in Avengers 2*
Really the Tony angle is the most suited for Ultron. I've seen some posts on other places giving a possible theory of busted Iron Man Armor with corrupted J.A.R.V.I.S. AI.
There's PLENTY of busted Iron Man armors after IM3...