I'm not a doctor...but I do have testicles. So let me see if I can help.
#1 If you're having severe pains and they keep happening. You must see a doctor. It could be a lot of things. But see one soon.
#2 One testicle is normally bigger than the other. Sometimes much bigger, which is noticeable more later in life and during physical activity or warm weather. This is the body's way of making sure they "dangle" at different heights so that they don't crush each other.
#3 The testicles aren't perfectly smooth. There is a part where the tubing attaches to the testicle and that may feel like a lump. However if you think it's more than this. See #1.
Again, don't freak out too much...but if you keep having the pain, get to a doctor! Tell your dad, he'll understand. All you need to say is, "Dad, I'm feeling a lot of pain in my groin, I'd like to see a doctor". Let us know how it works out.