Normal or Depression?

Hi. I 27 weeks along and I'm really struggling with my own negativity. I'm almost always in a bad mood now. I put on a happy face while at work but I lose sleep over things people say or do at work. When I get home from work I'm over tired and grumpy/lazy. I have two other children that need my love and attention and I'm finding it really difficult to give them the attention they need. Even if it's getting up to get them a cup of milk. I'm feeling guilty about it which in turn makes me feel even worse. I feel like I'm on my own island isolated. Is anyone else sharing the same struggles. Is this normal w a thirdnpregnancy. Or should I call my dr.  I'd like to just feel a little bit of happeniss but all I think about is when is this going to be over.