My first suggestion is to ask around your family--like your mom, someone discreet--and see if anything has changed in Grandma's situation lately. Maybe her income is drying up, she's been ill, she's becoming forgetful, etc.. It doesn't sound like you've done anything to alienate her (like not sending TY notes), so perhaps it doesn't really have anything to do with you personally.
Second, you said your son was born in November. Is it possible Grandma meant the birthday gift to also be a Christmas gift? If she's tightening her belt due to economic factors, she may have decided to just combine the holidays.
Third, you said she sent a Christmas card in 2012, so personally I don't see that as anyone being cut off. Do you have an otherwise good relationship with her--phone calls, emails, visits, etc.? It may just be that for whatever reason (again, nothing to do with you personally) she's decided to cut down on gifts altogether, without severing any relationships.
I think it's understandable to be hurt and confused, but to me it seems like there could be several explanations that aren't meant to be personally hurtful, so I think I would look into those first.