This is my theory of what happened in the wormhole or at least something I would like to see. According to Marvel, THANOS is "capable of space flight, force field projection, teleportation, time travel and movement through alternate universes."
So imagine if when Iron man crossed over the wormhole he met with Thanos and somehow made a deal with him, his life for something in return... Because the question "how did you get out of the wormhole supposes that it was impossible for him to get out of there alive, unless maybe he had help.
Or maybe Tony Stark saw a projection of the future along side Thanos, a terrible future where Thanos has succeeded in his plans and where earth has become a morbid planet with everyone under the Rule of Thanos.
There is something that is subconsciously really troubling Tony Stark a man who usually does not worry at all about almost anything, so logically it has to be really big and really bad.