what's a normal level of depression/sadness during pregnancy?

I know if you're feeling depressed you can ask your OB/midwife for an antidepressant or something to help, but how do I know it's not just normal? I don't feel sad constantly, but several times a week I just feel like no one really cares about me (usually when no one answers my calls or seems to care if I need anything) or like I just want to lay in bed. I've always felt this way so I'm not sure if I've just always had depression or if it's just normal mood stuff everyone experiences.