Certainly unfortunate and I don't blame you for being sensitive about it.
Who instigates the card-signing? Is it always the same person, or does it just depend on the recipient?
There are a lot of things that could be going on here, some of them less hurtful than others: Maybe the "card person" knew about the senior person's loss, but didn't hear about yours until much later? Maybe the people who know you well aren't the card-circulating type? Maybe the card-person was out sick or overwhelmed with work and nobody picked up the slack?
In any event, it's normal and not "over sensitive" to feel slighted, but if you otherwise like your job, it's better to try and attribute it to something impersonal/unintentional, if you can. If there's a pervasive pattern that you feel overlooked, unappreciated, or "kept out of the loop" about many things, then maybe looking for another job would be a good idea.