Even though nothing was found on the tests that were done, you clearly still have a problem.
I would advise that you do not go back to the gym until things have settled down. If you have pulled a muscle or a tendon this will take time to heal so you would need to avoid some workouts. Your gym instructor may be able to advise you with regard to what exercises and equipment you should avoid doing.
Avoid lifting and carrying things until things have healed.
Make an appointment to see your doctor again. The pain may be referred from low spine problems, tendons, ligaments or possibility of a hernia. It would be a good idea to get your prostate checked out too.
I do a fair amount of weight training myself and having a medical background, I can tell you, based on the details on your post, it looks like a classic case of Inguinal Hernia.
This condition is caused by a weakening of the groin/abdominal region
where a small opening called "inguinal canal" gets stretched and consequently herniated, often by strenious activity like weight lifting.
If heriation is considerable, surgury is the recommended treatment option.
Other lesser possibilities include infection of the epididymus-the coiled vessel which carries sperm-, if your symptoms include low grade fever, painful urination, painful ejaculation or frequent urgency to urinate.
If swelling of the testicles occurs there's possibilty (not very likely in your case) of testicular torsion-an ishemic event where twisting of the sperm cord from which the testicle is suspended, causes a cut-off in the blood supply- often accompanied by nausea and/or blood in the semen.
Without a physical examination and further testing, it is only my educated
guess and my comments do not in any way constitute medical advice.
Please refrain from heavy physical activity and consider seeing your doctor again to discuss and explore the aforementioned suspicions, should your symptoms persist or worsen.
This sounds indicative of a neuropathic pain; a type of pain caused by damage to the nerves. It could be as simple as a nerve torn or otherwise damaged by strenuous gym activity. This type of pain is very hard to manage if it is severe. You should know that the pain may never go away fully. It's important to know that there are measures that can be taken to reduce neuropathic pain (such as pregabalin). Go see your doctor and mention the extent of your pain if it persists, and you are worried.