If you fought a waspy-thing in the swamp recently (can’t think of the name right now), you might be harboring baby ones that will burst out of your flesh and do some damage. Yeah, ew. IIRC, you get some nausea for a while before that happens. I think that resolves it, though (can’t remember for sure - usually stay away from those guys, as they really aren’t worth messing with).
If you’ve eaten raw wild vegetables or meat, you may have a parasite.
Either way, you need anti-parasitic medication or the all-natural equivalent (mugwort oil, I think it is). And there’s no reason to take lots of antibiotics (or almost anything else in this game) - one dose takes care of the problem, generally. Taking fungicide (instead of anti-fungal medication) for a fungal problem has a chance to fail, but when it succeeds, you get a message that tells you so, so you can take it until success, then stop.
Only exception I can think of is vitamins - those give a benefit to health over time, so taking those daily does help.