I have a friendly acquaintance at the gym who shares a birthday with me. On Saturday, she gave me a card when we went out with other girls from the gyms. I want to reciprocate the gesture (our birthday is tomorrow), but I don't want her to think the only reason I'm doing it is because she did it first.
If you don't want her to think you are only giving it bc she did then I would do something different. Maybe buy her a waterbottle and put it in a gift bag or something like that
Get her a card. Don't elevate this to now having to exchange gifts. Because if you give her anything - water bottle or what have you - next year she'll feel she needs to give you something more than a card.
As tomorrow is your actual b-day, a card is fine. it's not an afterthought. You just waited until the day of.