I get this hard knot/lump on bottom of right testicle that flares up, it comes and goes and is painful.

See answer: Symptoms consistent with recurrent epididymitis and the "lump" you feel is likely the swollen, tender, inflamed lower portion of the epididymis which is a sausage-shaped gland that lies adjacent to the posterior-lateral aspect of the testicle through which sperm pass from the testicle to the vas deferens. To verify and/or get a definitive diagnosis, best to see a urologist when lump occurs again.