Oh dear. I totally disagree with the HV's comments. If your DS was fighting off an infection he will have been using more energy as a result. Also, he might have been taking less milk even though he was feeding more, it's impossible to know.
Your supply won't just randomly reduce. Just keep feeding as usual, maybe don't let him go more than 4 hrs between feeds, and see how he is at the next weigh in. If you supplement with formula that will affect your supply, if it replaces a breastfeed that your DS would otherwise have requested.
I really don't understand why the first piece of advice isn't to try feeding more frequently.
