Periodontal pocket? Or something else?

Hi I've only gone back to the dentist after years of not going. Had a bad experiences as a kid and never looked after my teeth properly when I was young. Even though I haven't been to the dentist in 5 years up until two weeks ago over them 5 years I did brush twice daily. I didn't floss but have since bought a waterpik and I'm determined to really look after my teeth. I'm 40 now and my wife tells me I left it too late to start really looking after them. I need 4 if not 5 more fillings and already have loads from years ago. I don't mind the dentist and I dont fear going because the new dentist I found has put me at ease. But one thing that is really bothering me is that over the last month my gum at the side of one of my back teeth is really painful. It's almost as if the gum is slightly coming away from the tooth. When I brush the bristles go down into the gap between my gum and tooth and the pain is like an electric shock. It doesn't bleed and my gums aren't inflamed. I told the dentist about it and she said she wasn't concerned. So what could it be? Or should I get a second opinion. The thoughts of going to yet another dentist is just a nightmare too.