They look like pus filled pustules like little acne spots to me too.
I have had urticaria in the past from heat rash and the rash did not have little white heads. It is likely that you are sweating more and the pores are getting blocked up hence the little white heads.
It also looks from the photo that you have an oily skin, and these type of spots are quite common on that type of skin. Have you noticed at all apart from the sun, whether this is linked to round the time of your period?
Make sure that you have a good balanced diet and eat plenty of fruit and veg. Don't overdo it on the junk foods, like biscuits and chocolate and fizzy drinks. And drink plenty of water, 2-3 litres per day is recommended, but more if you workout or the weather is hot.
Use gently soaps something like Simple Soap and nothing that is perfumed.
Best wishes
I was told that our bodies change with age. I think its from the heat of the sun. Try wearing a higher spf than you normally do and see what happens. if it happens again you might want to go and see a different dermatologist.
All I can see is what looks like acne to me. Are you sure it's not you getting sweaty when you're in the heat and getting clogged pores/whiteheads/pimples as a result? Or maybe heat rash? If it is a photosensitivity reaction, then wearing dark clothing and sunscreen over the areas should stop it. If you already use sunscreen, maybe that's clogging things up. You can experiment to isolate different potential causes.
That's all I've got. I'm not familiar with a large range of dermatological problems, though.