Is this a sign of low milk supply?

It is not a sign of low supply. It's 100% normal for babies this age to nurse at night, sometimes more than once. Formula digests more slowly, which is probably why she sleeps longer after getting it. But know that if you keep giving her formula, that may cause your supply to drop, since she isn't nursing as often as she needs to. So skip the formula (unless needed for some other reason) and keep nursing. She'll begin sleeping the n ight again when she's able.