Hi inkay88.  Welcome to the forum.

I am not a doctor and I have no medical qualifications.  However, I've had intermittent testicular pains for over 30 years!  Sometimes a cause is found, sometimes not.

You were wise to get an ultrasound at A+E. This might not have found the problem but you can be pretty sure it's eliminated cancer as a possibility.  That in itself should be a huge relief for you.

From my own experience, a likely cause of pain is some infection.  I'm not suggesting an STD or anything like that, just some microbe that has evaded the body's immune system and taken up residence in your naughty bits.  The top of the testis is a very common place to get an infection, which goes by the tongue twisting name of epididymitis. Seeing your GP and getting some antibiotics should sort the blighter out, but from my experience, it may recur after a period of months or years.  You may find that you need repeated courses over the years. 

Or it could be some form of trauma, or you may never get to the root of it.

In any case, your next step is to visit your GP and take it from there.