For a month, the front of my big toe has been extremely sensitive. for two days, the front and side of my toe nail is visibly red. what should i do?

Ingrown nail: I think you have an irritation form your nail rubbing the skin, and ingrown nail. Sometimes these get better on their own but you might have to have part of your nail removed to get it to feel better. See a local Podiatrist for a better diagnosis and he/she can remove the part of the nail and make your toe better.

Find Help: Sounds like an ingrown nail that may becoming infected. I don't know why it occurred in the last 2 days, but too short or tight shoes may have made it worse. You need the side of the nail that is red removed, soaking in warm water with Epsom salts will help, possibly antibiotics needed if redness is not resolved in a few days.