First of all, occassional pain in testicles is common because of its anatomic constraints. Make sure that you do not strain them during the day by using suitably supporting under garments. If you had a late puberty, it may be also due to growth period of the testecles as some children do get some pain during this period, especially due to more physical activities with inadequate support.
Also, nowadays youngsters heavily use laptops, and other internet devices keeping them in their lap, and in such cases, it is also possible to virtually fry the testicles due to heat radiation and electromagnetic radiations, if you are using them, never keep them in your lap while working.
Negative blood and urine tests have confirmed you do not have a urinary infection. Since both testicles are having pain, it may be also due to some inflammation / infection of the prostate, which may not show up in the tests.
In any case, you should not take any chance, you need to consult a urologist and get a proper diagnosis and treatment.
Have you been checked for vericocele veins in your testicles? A urologist can do this a well. Could explain the pain your having.