Hi. I Have A Sharp Pain When I Bite On

Hi. I have a sharp pain when I bite on certain things (e.g. grains in bread or crackers) on my two back teeth. This has been happening for many months. At my last trip to the dentist, I mentioned this thinking that I may need a filling but he took xrays and said he could not see any cavities. He said it could be a cracked tooth and to see how it went, and see if I could tolerate it. The pain is like a sharp nerve pain that shoots into my jaw. I noticed today that there is a red ring on my gum at the inside base of my back tooth on each side of my mouth. Do you have any idea what might be going on? Thank you