I don't think it is. I'd avoid supplementing as it is very much supply and demand, if you aren't trying to nurse (by using the freezer stash) your body isn't getting the signal how much you need to make. I'd just have baby on your breast often, drink TONS of water, and have faith your body will bounce back. If you need to supplement, do it after baby has tried to nurse.
At this point in your nursing relationship, your breasts probably won't feel as full. This may have been gradually happening before your illness, but you pay more attention to it now. I have breastfed one to 4 years old, one (still going) to almost 2. I have breastfed through two pregnancies (well, 1 and 2/3) and my breasts do not ever feel full anymore. The only time I noticed the "full" feeling was in the first few months after birth. It definitely goes away by 6-9 months for me, and that is more than likely the case for you. Your body is more fine tuned to your babies needs and doesn't need to get engorged every time anymore. I'd be concerned if your baby is still mainly breastfed, and stops having as frequent of diaper changes, but if your baby is receiving other nutrition is hard to tie that to breastfeeding anymore. Your baby is probably old enough to clue you in if they are still hungry after a feed or are not getting enough. Listen to your baby, nurse often, and drink lots of water.