I have rashes on both arms?

It sounds like it could just be a sweat rash or something. You didn't really specify much, as there are many, many different types of rashes. So it could just be from the heat. Also, it doesn't matter if this isn't your first time in Mexico, because if you didn't "catch any bugs" while you were there the other time(s), doesn't mean you didn't get it now. It could ALSO just be a developing skin condition, BUT don't worry yourself too much about it. I strongly recommend making an appointment with a dermatologist or a clinical doctor (possibly both) as soon as possible when you return home (I assume this is a vacation?). STRONGLY, STRONGLY suggest it. That is the one thing that will for sure diagnose your rashes. Could be malaria or dermatitis, too, seeing as how you're down in Mexico...I hope this helped!! (: Good luck!! Emmy