I too was a fool for tennis, playing 4x a week after work and weekends for hours upon hours.
My wife emphasized the importance of cross-training, but my tennis game just felt too good to give up, even for a day or two. I understand the addiction.
Well, 2 years later and I had the surgery to correct the problem and I'm back to hitting again. What a torturous road it has been.
May I graciously suggest that you stop playing tennis for at least a week? Your body NEEDS recovery time in between workouts, so this is nothing more than giving yourself a much deserved break.
As for the pain in your groin - if doc #1 said there's a hernia there would have been a palpable bulge. The fact that doc #2 said there was no hernia leads me to believe you may have another issue going on, especially because you now have pain on the other side.
That's exactly what happened to me. The surgeon was ready to admit me, yet I had no bulge. MRI, CT Scan, X-ray...all negative. Pain came and went, some days so terrible even walking hurt.
I read this thread religiously looking for people who could relate to my pain. Sure enough, just about everyone here has posted because they have similar diagnoses from docs who don't know that there are issues affecting the groin other than hernias.
As soon as I read that you have pain in your upper leg I thought you may have a tear, aka athletic pubalgia. Search for this term and read about it...mostly occurs as the result of twisting/running from a static position (repetitively).
If you can even somewhat self diagnose your self and you think you may have it, then your road may have surgery written on it. I wish I didn't wait, but at the same time, these two years opened my eyes to the importance of strengthening the core muscles, & training beyond tennis so that you get in shape to play, rather allowing the sport to serve as your only means of fitness.
So, to answer your questions more directly...yes, back pain is associated with this injury (and it's likely not a hernia, it's a tear). So much of this is the imbalance of muscles and the disposition of your spine.
Pain can be nominal to downright dreadful depending on the day and yes, many of us (like me) had pain on one side for a few months, followed by the same pain on the other side.
You twist and turn and sprint and contort your body in ways that make you quite susceptible to athletic pubalgia...do some more research on it while taking a break. I'm afraid you may just have to start from scratch to retrain your muscles, as they are very stressed and too used to tennis only.
There's much more to write, email me at joebshoe at hotmail.com if you'd like more info. The best thing you can do is get diagnosed as early as possible.