Am I overreacting?

[04liquidS]

DS (9 months) spent the night at my in-laws house last night (he's spent the night there before). DH and I went over to pick him up today. He woke up from a nap about 5 minutes after we got there. DH went and picked him up from the pack n play and brought him into the kitchen, where I was. His whole face and part of his neck was bright red and covered in hives! I pointed it out to DH and MIL. MIL tells me that he had them since earlier and she thinks he's allergic to either the area rug (he was rolling and crawling on it, maybe she used something to clean it?) or maybe the sheet on the pack n play (no clue what laundry detergent she used on it). So from what I understand, he had hives for at least several hours and she didn't bother to call and tell me or DH

After about a half hour, all but 3 of the hives went away (he still had 1 on his face, 1 on his butt and 1 on his thigh) so I called the pediatrician that was on call and he told me to give him Benedryl and call the office tomorrow (it wasn't his usual pedi that I talked to). We also gave him a bath, to wash off whatever he was allergic to.

I'm fuming mad. I told DH that I don't want them to babysit anymore. I don't trust them now. DS had an allergic reaction and they didn't call us What if it had gotten worse and he had trouble breathing? I'm not overreacting, right? If they don't babysit, there will be no more date nights but I would rather not worry about DS than have a date.