Ungrateful or exhausted?

Let me start with an introduction. I am a 21 yr old breastfeeding mama with 2 kids. My oldest is 2. My youngest will be 3 months on the 8th. I am fortunate enough to be able to be a SAHM. My oldest goes to "school" from 8-3 and my youngest stays home with me all day. I am in love with breastfeeding I really am. My oldest was a preemie and we didn't get to breastfeed very long at all so I am soaking up every second of being able to produce for my youngest. She is pretty well exclusive to the boob. An occasional bottle here and there. My husband works hard for us. 7-3:30 sometimes 6-4:30 Monday-Friday with the occasional Saturday. My problem is I feel angry when he gets home and doesn't have to do or worry about anything. He helps don't get me wrong. We alternate cooking and he takes out the trash and he sometimes changes diapers. But almost always I do everything else. Which is understandable in our situation. Why do I get so angry when he gets to leave or invite a friend over and leaves me alone with the kids?? I am not trying to be an ungrateful woman. I am very thankful for having 2 very beautiful, healthy children and a husband that works so hard, but I can't seem to stop these feelings. I can't just pick up and leave. I can't just hang out with a friend and forget my responsibilities because if I do they won't get done. Am I exhausted or am I really just that ungrateful?