I had testicle injury a few years back but it only hurt for a day and there was no swelling or anything now i've low normal t levels is that the reason?

Probably unrelated: Your low normal t levels are probably incidental and don't have anything to do with your prior injury. Testosterone should be checked on two occasions and should be associated with physical symptoms in order to be diagnosed as low.

Probably not: Unless you had bilateral injury. Even in unlikely event that injury damaged your testicle, the other side usually takes over.

The: Testicle main purpose is to produce and mature sperm as well as testosterone production, another percentage by your adrenal gland. Trauma has to be significant to the area to affect t production, including burning, torsion or severe edema.