Naomi just got a crazy rash from something she touched (contact dermatitis). The doc said a dose of benadryl was fine, and it worked like a charm. She's 25 lbs, she got a 1/2 tsp of Children's Benadryl. I think it's pretty normal for kids to react to the acid in tomatoes.
How old is she? Natalie had the same thing happen between the ages of 1 and 2 when she ate red pepper hummus. Red skin all around her mouth that cleared up within a few hours of me cleaning her off.
A lot of times the acid in the spaghetti sauce (or any acidic food - strawberries, citrus, etc) irritates their skin when they're little, and it isn't a true allergic reaction. Did it clear up soon after you cleaned her up? And it wasn't systemic (whole body), just where it touched her skin, right?
If it hasn't happened before, or with any other tomato product (pizza sauce, for example), I wouldn't freak out or run to the doctor. Now, if it happens again more severely, all over her skin, maybe be more concerned, but for now, I wouldn't worry too much.
***just saw the above post*** I'm pretty anti-hydrocortisone cream for little ones. Especially on their faces. It can bleach their skin, sometimes permanently, so be careful with that. :\
Actually, I think Dot's probably right. Maya reacted to Ranch dressing in the same way. It was just the vinegar/acid, it cleared up right away with just a clean-up, and there was no need for worry or a doc visit. The pedi said to wait a few months before trying it again, and she was fine the next time.
I bet it is just a little contact dermatitis. If it was going to be anaphylactic, you'd have known within minutes of her eating it, at the most.
Go get some benadryl tonight or tomorrow so you can have it if it happens again. Think about other tomato products she's eaten (or anything else that may have been in the sauce). Anything like this before? New brand of sauce? Maybe something in it is just irritating her skin.
Come to think of it, S was extremely sensitive to the acid in tomatoes as well. Both on his face an around his butt I always knew when he had even a little bit of tomato because if it didn't show up on his mouth, I saw it when changing his diaper. He'd get red bumps all around his butthole.