I Have Some Bites On My Back, Starting From My

i have some bites on my back, starting from my shoulder leading to my armpit. the bites are red with what looks like a white pimple in the middle (or pus) What can it be? I woke up this morning with them and noticed them after I showered. Can it be a heat rash? I was wearing a long sleeve shirt and I woke up very hot in the middle of the night.