What Causes Testicle Pain?

Hello.
I have been experiencing some testicle pain lately.
To start from the beginning, in September I had some discomforting pain in my left testicle that lasted a few days and later started going away, in one week it was all gone.
For about a month now I had some dull pain in my scrotum again, at first I remember radiating more from left again but now it affects my right testicle that is naturally bigger than my left and hangs lower ( as my left was undescended and fixed 10 years ago ) only that I have a slight impression it never hanged horizontally low like now, gives me the impression it is swollen a bit, since it hangs this way, I measured it something like 4 cm long.
I can describe the pain as more discomforting when I sit down or stand up not moving, when I walk it seems to simply go away. Also the pain seems to radiate a bit to my right leg sometimes also when I sit down.