Yes, it's possible to have spasm without pain. For me, the pain going away is sometimes a sign that the spasm has gotten worse. What happens to me is, I get bad back pain for a while, when the muscle is only partly spasmed, or the spasm is coming and going. Then, if it locks up all the way, the pain there goes away, but I start feeling pain in other areas, like my hips and shoulders.
The easiest way to diagnose spasm is to touch the muscle. Muscles are supposed to be firm but soft. If it feels like a steel cable under your skin, it's spasmed!