For a year or so, a tooth has decayed and the sides broken off. on eating nuts today i now have a lump on my gum (inside) under the tooth. what is it?

Check out the lump!: It sounds like this problem has been going on for some time and it will not get better on its own! at this point the lump is telling you that the decay in the tooth is spreading, and needs to be addressed if you are going to be able to save the tooth and stop the infection from spreading further. It may already be too late to save the tooth, but you don't want the infection to ruin the bone.

See your dentist: The broken tooth may have led to chronic irritation, but there are many other possibiliites - so you should see you dentist.

Infetcion?: Eating nuts could be insignificant and coincidental. If this tooth has been decayed for a while, enough to break off some of the enamel, there is likely an infection deeper in the roots of the tooth. The lump on your gums could be the abscess that is trying to drain from the infection. Get this treated by a dentist right away, before it becomes very painful and more threatening.

Infection: Sounds like an abscess. Time to visit the dentist. Untreated tooth infections can be fatal.