It is difficult to differentiate skin bumps without seeing them however we can look at a number of possible causes.
A bright red bump which is raised can be a bacterial infection.
Insect bites also cause bumps and this can be from fleas, as well as mosquito's and other flying insects.
Flat bumps can be fungal infection, sweat rash as well as possible eczema.
Allergic bumps tend to be small and pink/red unless you experience urticaria where you get a large fluid filled bump.
Viral infection would cause small bumps normally, and these would continue to spread over the chest and abdomen.
Taking minocycline then you may be experiencing a skin reaction, especially with summer sunshine but there does not sound enough bumps to be a drug reaction.
As they appear to have started after your new job then I wonder if you are having fungal infection through increased sweating from the activities.
Because I cannot see the bumps I don't want to recommend a particular cream but if you pop into your local pharmacy then the pharmacist will be able to look at the bumps and confirm what the problem is. Otherwise you will need to see your doctor.