The one word
Sakit covers sick, ill, sore, painful and hurts. You do not need separate vocabulary for being unwell and for something aching.
Saya sakit. ‘I’m ill.’
Perut saya sakit. ‘My stomach hurts.’
Naming the part
Put the body part first and sakit after, exactly like any other adjective.
kepala ‘head’
perut ‘stomach’
gigi ‘tooth’
tenggorokan ‘throat’
punggung ‘back’
kaki ‘leg, foot’
tangan ‘arm, hand’
Sakit kepala. ‘Headache.’
Sakit perut. ‘Stomach ache.’
Note that kaki covers both leg and foot, and tangan covers both arm and hand. Indonesian does not split them, and if the difference matters you point.
The rest of what you need
demam ‘fever’
mual ‘nauseous’
pusing ‘dizzy’
alergi ‘allergic’
Saya demam. ‘I have a fever.’
Saya alergi antibiotik. ‘I’m allergic to antibiotics.’
Getting help
Apotek di mana? ‘Where’s the pharmacy?’
Saya perlu dokter. ‘I need a doctor.’
Ada obat untuk sakit perut? ‘Do you have something for a stomach ache?’
That last one uses ada from the essentials course, and it is how you actually buy medicine: you describe the problem rather than name a product.
Rumah sakit is hospital, literally ‘sick house’. Worth knowing before you need it.
What you’ll actually hear
Pharmacists in tourist areas often speak enough English, and many will hand you something after hearing one word. Saying sakit perut rather than miming will still change the interaction, and in a village it may be the only option you have.