Specifically for your back, you need to do a course of back exercises to help you get "over the hump".
These exercises seem rediculously easy to do, but will help get the para-spinal muscles that are tense to stretch out, and help build strength in the tiny back muscles that oppose the action of ones you have "strengthened" from tennis activities.
Your back - like all your muscles - have to be evenly balanced between those that push and those that pull - or else the strong ones will pull your back out of alignment and result in pain.
Slide show: Back exercises in 15 minutes a day http://www.mayoclinic.com/health/back-pain/LB00001_D
Most find that if they are later doing more major exercise like squats that involve the back, that these "ridiculously easy to do" exercises no longer become necessary.
(Most - but not all.)