Testicular Cancer/ Cyst?

Hi, about 2 months ago I had pain in my testicular/epididymis region. I went to the doctor to get it checked out. He did a examination and said everything felt fine. He also sent me to do an ultrasound. I went in for the results he said everything looked fine that there was no cancer. They did find i had a small cyst on my left testicle. He advised me to do self examinations every so often when i shower to look for lumps. I had never done self examinations and after the first one i discovered a small smooth lump smaller that a pea on top of my right testicle which did not come up in the ultrasound. I can move it around somewhat. I went back to the doctor because i have been experiencing pain on the little lump. The pain is only present in the lump and goes away quickly. I asked him to make sure if any cancerous evidence came up on the ultra sound and he said everything came out fine. That ultrasounds are pretty much always right. I have also experienced lower back pain and have had some soreness in my legs but it is possible it might be from work (UPS). He said to come back in 6 months for another ultra sound. Im still a little nervous about that lump though can it be that the ultrasound didn't show full results? The doctor also told me I should be ok, to come in before the next ultrasound only if I found the lump is bigger than a pea or if the pain is so much i can take it. Im still kind of nervous though. Any thoughts or ideas?