I forget, are you on Lamictal (and had any recent dose changes or have been taking it inconsistently)?

I wouldn't think it's too serious, but I'm just saying if I were on Lamictal, and how I take it's changed, and got a rash all over my arms and chest, I'd freak. I know SJS is really rare, but my first NP that prescribed it instilled the fear.

I know you've already had a couple other replies too, and I'm going to say allergy to something else or stress is more likely. We're getting fall changes here and I tend to get rashes just from the air changes (drier, dust from heat kicking on, different pollen, etc.) so that could be it.