How new is the calf? It's pretty common. Some of my cows I can get right next to their calves after they have them and check the calf out and do pretty much anything with and they will just watch real close and talk to their calf while I'm working with it. I've got a couple cows that I don't get near their calf for a few days unless I can help it, cause they will get in my pocket if I get too close to their calf. Ordinarily I can get right next to them in the field and they are calm as they can be, but once they calve it's like they flip a switch and they are in momma mode. This is good against predators, but not great when it's me they are chasing off. I always keep some type of stick/pitchfork with me and close to a way out just in case they decide to get after me, cause they can all be unpredictable after calving.