If the signs started after bathing with the head and shoulders then that may be the culprit here. Some of those human shampoos can really dry out the skin and they can have reactions to it. I wouldn't use this anymore.
If this is a treat that she's had before, then it's not likely the cause. Dogs can have food allergies, so it's possible. I would doubt that it would come on so fast though, usually it's a less sudden onset if a food allergy and we more likely would see vomiting and diarrhea.
Benadryl- you can use regular human benadryl. Give 1 mg/lb. If she is under 20 lbs then you can use the childrens liquid benadryl- I think it's 5 mg/mL but not sure, you'll have to read the box.
If it's fleas, then you can get a topical flea product like Frontline which is now over the coutner at pet stores. You usually will see flea dirt on them- it looks like little black "dirt" looking specks. When you run them under water it turns red as it's flea feces (digested blood). If you see this, then I bet the whole thing is flea allergy. In that case, yes wash all the blankets that she's been on and be sure to vaccum the rugs where he lays as well.
So, start with the bendaryl for the itching. Hopefully the bumps go down as well when she stops itching so much. If in 2 days from now it looks the same or worse, then I'd get her to a vet and get it looked at. I know they can be expensive, but 2 days of itching would be my limit.
As for bathing him again- it might be a good idea to use a dog shampoo and get the other stuff off him. A tearless baby shampoo is an ok alternative. Sometimes it can be hard to wash all the shampoo out completely so it dries and can irritate- especially if it was a human shampoo.