hormones are crazy things. sometimes they behave and sometimes they don't. i didn't have any major problems with acne when i was in high school. now i'm almost 24 and having some serious break outsm especially on my shoulders and back. so there you go.
if you have been sweating a lot lately, it could contribute to breakouts. it's really important to shower and change into clean clothes after working out so that the sweat doesn't stay on the body. also, make sure that when you do wash, don't scrub the broken out area too harshly because this could just further irritate the skin.
you said you are from australia so i'm not sure how the weather is down there this time of year but it's probably warm, right? i have noticed that i tend to break out when it is really cold and my skin gets dry. sometimes it overproduces oils to compensate for the dryness and voila! breakouts.
i would use a body wash formulated to treat and prevent acne. some good ones are Neutregena Clear Skin body wash and Phisoderm makes a good one as well. Don't over treat with products containing salycylic acid (most OTC acne gels, etc) because the medicine you are taking is probably already drying the area out.Also, it wouldn't hurt to call your dermatologist and explain to them what is going on and ask if it is a possible side effect from getting used to taking the medicine.