I have burning under my big toes. what could it be?

A few things: Burning pain under the big toe (hallux) can be due to a lot things- neuropathy (from medical problems like diabetes, rheumatoid arthritis, poor circulation), arthritis, osteoporosis, underlying bone deformity, callus, wart, foreign body or another injury. A podiatrist can help with the diagnosis and offer treatment options.

There are many: Possible causes. Nutritional deficiency, side effects of medication, small nerve fiber disease, nerve compression, low back issues, some systemic diseases like diabetes can cause this as well. I would recommend you see a doctor to help differentiate which of the above it can be.

Possible neuropathy: Neuropathy can happen as a result of trauma, or nerve damage due to the consumption of alcohol or certain medications. Symptoms can range from a numbness and tingling sensation, possibly a burning sensation, to an uncomfortable painful sensation of the extremities. Get evaluated by a professional to determine the exact cause in order to get the appropriate treatment.