Confused about my sons reactions

My 12 year old son has been having a skin reaction 4 days in a row. The first time he got off of the school bus with swollen hands and a rash on his torso. The rash was still spreading and we gave him benadryl. The benadry knocked it out pretty quickly. We had no idea what caused it and assumed he came in contact with something new at school or on the bus. The next day (Thursday) he did not have a rash or anything when he got home so we thought it was a freak incident. About an hour after getting off the bus a rash began on his body and covered his torso, legs, and neck. We gave him benadryl again and it knocked out the rash like before. Now we're really trying to figure out what's causing it and our main suspicion was his new medication (vyvance). I let his psychiatrist know about the reaction and she wasn't sure if it could be the medicine either.

He had been on that particular medicine for 2 weeks before the first reaction but had been on other stimulants for the past 5 or 6 years. He took 30mg for a week and 40mg the second week and was actually about to start 50mg. Instead of giving him the 50mg on Friday, I gave him a 30mg and about 2 hours after he got home from school a rash developed again, although much smaller than before. I was on his torso, neck, and face. I took him to urgent care and they noticed that he had gotten a few vaccines a week prior and apparently the combination of 2 of those vaccines (tetanus and one i forgot) sometimes causes a reaction like he is experiencing but after she went out to look something up she came back and said that it was probably the medicine because of the type of rash (not really raised off of the skin). She suggested not giving him the vyvance until I see the psychiatrist. I did not give him the vyvanse this morning and everything was fine until around 9pm when he developed a rash in both shoulders, much smaller and later in the day than yesterday's rash. Now I am confused again. The vaccines were last Friday and the rashes started Wednesday and have progressively gotten milder each day. I can't see the psychiatrist or his pediatrician until Tuesday since it's a holiday. I'm thinking I'll take him to the ED tomorrow if he gets another rash to see if they can help pinpoint the culprit but I'm at a loss right now. Nothing else is new.