You would probably have to experiment to find out - if the rash went away within a few hours but then came back the next time your child ate tomato, it might be an intolerance or allergy.  However I believe (but don't quote me!) that with allergies babies are more likely to get a skin reaction around the mouth rather than on the chest. 

It might just be a coincidence that your baby had a rash for some other reason (viral rashes are very common) and you noticed it the same day your child first ate tomatoes.

you will find red rash around the mouth from foods that are acidic like tomatoes. Sometimes you'll find this to be a contact reaction only though. Some kids that may get a rash when eating raw tomatoes will not if you feed them spaghetti with tomato sauce. 

My son had the same reaction when he was younger, but no longer.