Pimple like rash

Hi,

I'm a female, age 29

I was at the beach from 10:30 am- 4:00 PM and it was super hot out. Like a 100 in NJ. Very humid.  I applied an spf of 4 only.  I didn't really burn, just a tad bit pink.  I have a nice base tan. I was in the ocean for a good part of the day.

My back began to itch while at the beach, didn't have any kind of rash though.  I woke up the next morning to a rash on my chest and upper and mid back.  It looks like pimples (whiteheads) as, there was a head to each pimple.  It didn't itch, but is sensitive to touch.  It's two days later, my chest is starting to clear up, but my back has the pimples still, but no itching since the beach, before I even had the rash....

I have had this happen once before when I was away on an island.  

Can you please tell me what causes it? It has to be from the sun since this happens only when at the beach....