I have had a dull pain in my lefy testicle and found a lump which I haven't noticed before. What are some possible causes?

Need evaluation. Could be several causes of pain and lump in the testicle, but the most concerning is testicular cancer. You need to be evaluated by your medical doctor as soon as possible to have an exam and possible ultrasound. Other causes include infection, spermatocele, varicocele.
R/o tumor. Growths on the testicle must be checked to be sure they do not represent a cancer. Growths on the other structures in the scrotum are more commonly benign such as spermatocele, varicocele, cyst, granuloma, or other growth. An ultrasound is often very helpful in sorting out exactly what the anatomy looks like and can guide future therapy. Tumors rarely cause pain.