The part below my big toe nail hurts a lot, what could. it be?

Ingrown toenail?: It's impossible to give you a diagnosis over the internet. By far, the most common cause of pain next to or near a toenail is an ingrown toenail. But there are other causes of such pain. The best advice i can give you is to see a podiatrist. If it is an ingrown toenail, the procedure to fix this is fast, simple, easy and painless with no disability. Hope this helps. Good luck!

Many things: If the pain is at the proximal nail fold (skin at the base of the nail plate), you could have an infection, especially if it is red or draining. If the pain is further back near the joint, you could have a hyperextension injury or compensation, causing pain during walking or running. If this is the case, try a stiffer shoe, avoid barefeet, flip-flops or flexible shoes.

A few things: It might be due to nail fungus, an ingrown nail, bone spur under the nail, or injury. See a podiatrist to get a specific diagnosis and pain relief.