[CROSSPOSTING - Sorry]
First of all, there is a similar thread going on here: https://www.breastnexum.com/showthread.php?tid=31565
It makes sense to keep an eye on both.
If you search online, the issue of testicular pain is common but it has never really been studied. The pain is a dull ache, like the aftermath of being kicked in the balls, but not immediately after the kick. Also it could feel similar to when someone holds your balls and squeezes a bit too much: no jolt of pain but a serious discomfort.
The pain seems to go away after a few weeks, and can occasionally come back if you stop and restart taking estrogens. After the pain goes away it's not uncomoon to notice that the testicles are smaller than they were, and they never go back to full size again. They won't lose their function, sperm quality seems to be just as good as before in early stages of NBE or HRT, and sex drive is back to normal when stopping the estrogens.
It's an interesting phenomenon and it's a clear indicator that the estrogens are working.