Ugh, and we have a new one too ATM

AND it sounds similar!
http://babywhispererforums.com/index.php?topic=242029.0Okay so only based on our history, and it does sound very similar, though our rash had more whip like white marks through the rash - and raised, I think it is most likely viral.
What I did learn after last time was that you can have a histamine allergic reaction to a virus! - these can last a while too...like weeks.
If the antihistamine helps, and if your LO is generally well, and nothing else has changed then I would give it up to two weeks before panicking. Only because drs etc just can't and won't really do much before tis timeframe unless it is causing LO discomfort/making them unwell, or appears to be a dangerous rash.
As I say this, I have posted in a panic

hugs