Yes, you are being oversensitive. Unless you go out AFTER work, on a regular basis with your coworkers, they are NOT your friends. They are co-workers, plain and simple. The fact that YOU choose to buy them gifts and cards does not obligate them to return the gesture. And their intent may have been to get you to stop. They may find you overly involved, and welcome you backing off.
I don't blame you for being hurt because they didn't reciprocate. But, you didn't say that they have acknowledged your birthday in the past or if any of the other women get cards or gifts for other coworkers. It may be, like the previous poster suggested, that they are not as interested as you in celebrating everybody's birthday. It does seem odd that the company didn't give you the requisite flowers, unless you were the person that alerted the boss/responsible person that a birthday was approaching. And not everybody thinks that 40 is a "huge" birthday that needs a lot of notice.
I agree that you shouldn't continue to do something for their birthdays because gifting and celebrating should be given or done with no expectations (and that can be hard).
I used to work in a small office of women and they had a tradition of everybody pitching in for a card and store-bought cake which came to a few dollars per person. The task of remembering whose birthday was coming up and organizing the rest of it usually fell to one or two of the women (I became one of the them). If neither of us did it, I'm not sure that it would have happened...everybody was willing to pay a few bucks but most didn't want to be bothered with shopping for card or cake. So in your case it may be more a matter of inertia than deliberately snubbing you.
Its been a few months and you're still angry about it....try to let it go and accept that this kind of birthday celebrating won't happen with these coworkers.