Is it possible to develop a hemorrhoid from lifting dumbbells?

Yes. Frequent, heavy lifting can result in hemorrhoids. This increases the pressure in the veins in the pelvis, which can enlarge them. This can eventually result in hemorrhoids.
Possibly. Straining of any kind including constipation or difficulty urinating, can cause increased shunting of blood to the pelvis and engorgement of the hemorrhoidal veins, usually more so to the external hemorrhoids. If they swell, the best treatment is warm soaks and bed rest for 24 hours. If they rupture or thrombose and are causing a lot of pain, may require lancing.