Pym Poisoning
In the weeks following the battle to free the various women from the Deviants, Steve had seen almost no super-heroic action.  It was as though the world of crime had decided to take a bit of a holiday break.  He'd continued to take his regular doses of growth formula, just in case, but had only had one call to change size during that time.

Now, though, he entered the training room, ready for a giant-sized work out.  He concentrated, getting ready to push himself to his nominal fifteen feet fighting height.  He felt the usual tingle run through his body and then...  pain.  It started in his guys but quickly spread to the rest of his body, feeling like his every nerve was on fire.  He couldn't move, his muscles felt locked, couldn't even cry out in pain or cry out for help.

He saw the floor coming up to meet him (or was he coming down to meet it?  He couldn't tell) but could not do anything to stop it.  He hit the floor with a hard thud.