try switching laundry soap and see what happens... Make sure to wash all sheets and his clothes... What clothes is he wearing when it happens... Any snaps, buttons, appliques near the hives? Were they onsies (nothing covering the legs.) Did he have socks on or was his skin in direct contact with bedding, carpet, etc... It also could be a reaction to a bug bite. (my guys welt up when they get mosquito bites)
Also could be a slight reation to the formula (If he's on formula)
If its only happening on his left side consistantly I doubt it would be a reaction to shots or food...More of an enviromental thing... I would keep a log and note what he was doing for about an hour before the welts (If you can remember)... What rooms, what toys, etc...
I know reactions like that are scary... My 4 month old had a reaction to formula and welted up several times... They are so small and can't really say "mom... Help me... I don't feel right"... Thankfully with my guys all we had to do was trial and error new formula (ended up with soy)... His welts were all over his body though and HUGE!!!
I hope you figure out whats happening...
And if you do have to go to the hospital again remember that the squeaky wheel gets the oil... So go up to the desk every 15 minutes and ask them if the dr could just come out and LOOK BEFORE the welts go away... Take pics of the welts to show your dr...