Sometimes they need to be put down firmly in crosswind/gusty conditions or short runway etc. It doesn't necessarily mean anything was done wrong because the landing felt a bit on the hard side. In fact a gentle landing can be more dangerous than a firm one in some situations.
If none of the oxygen masks dropped, it probably wasn't very hard
Now if you saw a rubber jungle, it might well have been quite hard...
If the subsequent flight departed on time, then it's likely that it wasn't very hard because a proper heavy landing would have needed an engineer to check it (it's all recorded...).
Quite a few people seem to tense up their back when a firm landing had happened (or in rough sea if they are on a boat) and it can feel like the back has been jarred but not necessarily so. A bit of a sore muscle from tensing up is quite often the cause...