Aching pain in left testical

Over the last two days I have a dull aching pain in my left testical? Any ideas what it could be? Could it be STD related??

It could be a std if you had unprotected sex but it could be another bacterial infection of either testicle itself of epididymis.

Do you have any other symptoms accompanying the aches? Any lumps, swellings, redness, tenderness, etc…
It could be just a transient thing that would last for a few days and just go away on its own and never come back.

Did you do any heavy lifting, stretching, or had a trauma to the area lately?
If the ache continues for another couple of days, if it gets worse or you develop additional symptoms, I suggest you see a doc.