Can subcutaneous hematomas cause little to no pain?

It can happen.. Often, small bleeding/hematoma or bruising, especially in elderly on aspirin/blood thinners, do not cause any significant pain/discomfort. Many don't even know how they got the bruises etc.. The skin itself is sensitive, but deep under skin there is less nerve endings and thus trauma/bleeding in deep area may not cause much pain at all, unless large and pushing on other tissues. Consult doc..
Of course, but. It depends on the location and size of the hematoma. Hematoma refers to an actual collection of blood rather than a bruise, which is more like a soaked sponge, with tissues being the sponge. Both will eventually reabsorb on their own, but hematomas may do better and resolve faster if aspirated with a needle by an expert in surgical anatomy.