Is burning in your feet and 3.5/10 leg pain (both legs) a sign of nerve damage, peripheral artery disease, diabetes, or is it just caused by low blood flow from sitting down too much?

Low blood flow very unlikely at age 19 (and never by sitting too much). Sounds more like peripheral neuropathy (diabetes?, heavy metal?, toxin exposure, B12 deficiency? Etc). Check with Internist of (preferably) Neurologist

Agree with Dr Lentnek, as the most likely cause of your foot burning would be a problem with peripheral nerves. See your primary physician and get examined, but likely will need a neurologist. An EMG test may be helpful if question of tarsal tunnel syndrome or large fiber neuropathy, but if negative, a skin punch biopsy may confirm small fiber disorder.