Hi there!
Well, without a clinical evaluation it would be difficult to determine the cause of your symptoms. It is normal to have slight discrepancy in the size of the two testicles. Possible causes of the symptoms that may need to be considered include infections, inflammations, hernias, hydrocoele, varicocoele, growths/ masses etc. I would suggest getting this evaluated by a urologist for an accurate diagnosis and appropriate management.
Hope this helps.
Take care!
And I don't feel a lump on it either. Kinda hard to tell but I'm 99% sure there isn't.
Also one testicle (left one) feels harder and bigger than the other. It's the one that brings on pain sometimes.