My Tattoo Feels..............

I've had a tattoo for a little over 5 years. Got it at an established shop in New Mexico. No problems at all, healed great. All went well for about 2 2-1/2 years, then every now and then the whole outline raises. It feels just like it's a new tatt. I doesn't hurt, but, I've asked dozens of people, including several artists, and no body can explain why. It usually lasts about 5 minutes and then it just goes away. Any answers would be greatly appreciated........