I know what you mean...it is hard to juggle relationships when you have a young child and friends and family without children won't understand until they do have kids. My advice to you is to stick to your guns. Tell your friend that you have to allot time for everything you do every day in order to get everything done. Tell her that you cannot talk to her in the morning because it just isn't a good time but that YOU will call HER in the afternoon whenever you have the chance. Make it clear that you value her friendship, but that things have changed and you have OTHER (meaning that she and your relationship is also a priority that needs a certain amount of time allotted to) very important priorities to see to and juggle. The last thing you need is STRESS from a friend. Friends are there for support and understanding, not to stress you out more during tough times in your life. Maybe if you very gently tell her that you need her support, not animosity, she will get the point.
I don't think you are overreacting. I know how hard it is with young children and many other people have no respect for what we deal with! My one friend used to call me all the time, several times a day, often when my son was just about to nap (which would wake him up and make my life miserable) so I had to shut my ringer off. I would call her back when I got the chance but I let her know right away that I would have to call her, not the other way around. I don't know if she likes it but she has to deal with it to remain friends. I have so many other things to stress about, I don't need more stress from a friend. chatting on the phone is no longer an easy thing to do. It takes time, energy, takes away from valuable alone time, the works. And you are probably losing sleep time too just to give her a call, which is the ultimate sacrifice when you have a young one. Just be honest with her and let her know that she IS a priority on the afternoon list, above all else. To sacrifice naptime just to talk to someone is the ultimate honor!