My 2 yr old got some silver caps on in August but one fell off like a month ago and i went to have…

My 2 yr old got some silver caps on in August but one fell off like a month ago and i went to have it put back on right away. But as i was brushing his teeth tonight i noticed his gum behind that tooth is redish and yellowish color. Im going to call his dentist tomorrow because i think its an infection. Im really scared and cant sleep just thinking about what it could be and how serious it is. Anyone that has had this happened to them?