Any number of things could be causing it. My daughter's skin was temperature sensitive; she'd look like she's gotten slapped after her nightly bath, it was really odd. It could be rosecea, eczema, vitamin deficiencies, a simple reaction to a lotion or soap you are using....the list is endless. I do know that if you are concerned, seeing your doctor is always a good idea. Unless I'm 100%, I never self diagnose; sometimes the doc agrees with my suspicions, and sometimes it's something I would have never guessed. I would suggest you take a picture of it when it occurs, and schedule an appointment with your doctor. Ask them to do a CDC just to make sure you're not short on iron or any other vitamin (although your doc will likely do this anyway), and see what he/she says. My fiance has eczema on his forearms too, though he didn't know that's what it was til he saw his doc; now he has cream to use that helps alot. I unfortunately have small eczema on - of all places - my areola. Yes it majorly sucks. and it flares up under stress big time, and in the winter time. And again, I found that out by my doc. So get yourself tested - it's always better to be safe than sorry.
Hope this helps!