It does not matter ,that the first time, you did not get infected ,in that occasion.
But what you probably have, is a Diving Suit Folliculitis, the suits ,used for scuba diving, can create a moist, occlusive enviroment that predisposes skin to the growth of Pseudomona Aeruginosa (a bacteria), that produced these sudden pimples , on the back of your neck and on your back.
You will need going to the doctor,to see if he thinks that you will need antibiotics ,to clear the infection , like Ciprofloxacin ,by mouth,twice a day.
In the meantime, you can use of acetic acid 5% , 4 times a day, for 20 minutes.